Support for meta and hyper keys added

Support for the compose key has also been added but it's just the
button code for now, no support for actually composing stuff.
There are plans for that in a fixme.

examples/printbuttons/main.go

@@ -0,0 +1,41 @@
+package main
+
+import "os"
+import "image"
+import _ "image/png"
+import "git.tebibyte.media/sashakoshka/stone"
+import _ "git.tebibyte.media/sashakoshka/stone/backends/x"
+
+var application = &stone.Application { }
+
+func main () {
+	application.SetTitle("press any key")
+	application.SetSize(8, 1)
+
+	iconFile16, err := os.Open("assets/scaffold16.png")
+	if err != nil { panic(err) }
+	icon16, _, err := image.Decode(iconFile16)
+	if err != nil { panic(err) }
+	iconFile16.Close()
+	iconFile32, err := os.Open("assets/scaffold32.png")
+	if err != nil { panic(err) }
+	icon32, _, err := image.Decode(iconFile32)
+	if err != nil { panic(err) }
+	iconFile16.Close()
+	
+	application.SetIcon([]image.Image { icon16, icon32 })
+	
+	application.OnPress(onPress)
+	application.OnRelease(onRelease)
+	
+	err = application.Run()
+	if err != nil { panic(err) }
+}
+
+func onPress (button stone.Button) {
+	println("press", button)
+}
+
+func onRelease (button stone.Button) {
+	println("release", button)
+}