Changed the API to make it handle keynav better and be simpler

I have yet to actually update the code to reflect these changes.
I will do this shortly.

tomo.go

@@ -4,7 +4,6 @@ import "image"
 import "errors"
 import "image/draw"
 import "image/color"
-// import "git.tebibyte.media/sashakoshka/tomo/iterator"
 
 // Image represents a simple image buffer that fulfills the image.Image
 // interface while also having methods that do away with the use of the
@@ -15,14 +14,35 @@ type Image interface {
 	RGBAAt (x, y int) (c color.RGBA)
 }
 
-// Canvas is like Image but also requires a SetRGBA method. This interface can
-// be easily satisfied using an image.RGBA struct.
+// Canvas is like Image but also requires Set and SetRGBA methods. This
+// interface can be easily satisfied using an image.RGBA struct.
 type Canvas interface {
 	draw.Image
 	RGBAAt (x, y int) (c color.RGBA)
 	SetRGBA (x, y int, c color.RGBA)
 }
 
+// ParentHooks is a struct that contains callbacks that let child elements send
+// information to their parent element without the child element knowing
+// anything about the parent element or containing any reference to it. When a
+// parent element adopts a child element, it must set these callbacks.
+type ParentHooks struct {
+	// Draw is called when a part of the child element's surface is updated.
+	// The updated region will be passed to the callback as a sub-image.
+	Draw func (region Image)
+
+	// MinimumSizeChange is called when the child element's minimum width
+	// and/or height changes. When this function is called, the element will
+	// have already been resized and there is no need to send it a resize
+	// event.
+	MinimumSizeChange func (width, height int)
+
+	// SelectionRequest is called when the child element element wants
+	// itself to be selected. If the parent element chooses to grant the
+	// request, it must send the child element a selection event.
+	SelectionRequest func ()
+}
+
 // Element represents a basic on-screen object.
 type Element interface {
 	// Element must implement the Image interface. Elements should start out
@@ -37,17 +57,21 @@ type Element interface {
 	// element contains other selectable elements, it must return true.
 	Selectable () (bool)
 
-	// SetDrawCallback sets a function to be called when a part of the
-	// element's surface is updated. The updated region will be passed to
-	// the callback as a sub-image.
-	SetDrawCallback (draw func (region Image))
-
-	// SetMinimumSizeChangeCallback sets a function to be called when the
-	// element's minimum width and/or height changes. When this function is
-	// called, the element will have already been resized and there is no
-	// need to send it a resize event.
-	SetMinimumSizeChangeCallback (notify func (width, height int))
+	// If this element contains other elements, and one is selected, this
+	// method will advance the selection in the specified direction. If no
+	// children are selected, or there are no more children to be selected
+	// in the specified direction, the element will unselect all of its
+	// children and return false. If the selection could be advanced, it
+	// will return true. If the element contains no child elements, it will
+	// always return false.
+	AdvanceSelection (direction int) (ok bool)
 
+	// SetParentHooks gives the element callbacks that let it send
+	// information to its parent element without it knowing anything about
+	// the parent element or containing any reference to it. When a parent
+	// element adopts a child element, it must set these callbacks.
+	SetParentHooks (callbacks ParentHooks)
+	
 	// MinimumWidth specifies the minimum amount of pixels this element's
 	// width may be set to. If the element is resized to an amount smaller
 	// that MinimumWidth, it will instead set its width to MinimumWidth.
@@ -64,15 +88,35 @@ type Element interface {
 // be explicitly shown with the Show() method. If it contains no element, it
 // displays a black (or transprent) background.
 type Window interface {
-	Element
+	// Adopt sets the root element of the window. There can only be one of
+	// these at one time.
 	Adopt (child Element)
+
+	// Child returns the root element of the window.
 	Child () (child Element)
+
+	// SetTitle sets the title that appears on the window's title bar. This
+	// method might have no effect with some backends.
 	SetTitle (title string)
-	SetIcon  (sizes []image.Image)
-	Show     ()
-	Hide     ()
-	Close    ()
-	OnClose  (func ())
+
+	// SetIcon taks in a list different sizes of the same icon and selects
+	// the best one to display on the window title bar, dock, or whatever is
+	// applicable for the given backend. This method might have no effect
+	// with some backends.
+	SetIcon (sizes []image.Image)
+
+	// Show shows the window. The window starts off hidden, so this must be
+	// called after initial setup to make sure it is visible.
+	Show ()
+
+	// Hide hides the window.
+	Hide ()
+
+	// Close closes the window.
+	Close ()
+
+	// OnClose specifies a function to be called when the window is closed.
+	OnClose (func ())
 }
 
 var backend Backend