Add sync.Locker interface to Drawable interface

Many widget implementations will want to update asynchronously, so now termui performs locks a widget when drawing. Users should call `Lock()` on their widgets when performing an asynchronous update.

CHANGELOG.md

@@ -6,6 +6,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## 2019/03/07
+
+### Changed
+
+- Added sync.Locker interface to Drawable interface
+
 ## 2019/03/01
 
 ### Changed

block.go

@@ -6,6 +6,7 @@ package termui
 
 import (
 	"image"
+	"sync"
 )
 
 // Block is the base struct inherited by most widgets.
@@ -25,6 +26,8 @@ type Block struct {
 
 	Title      string
 	TitleStyle Style
+
+	sync.Mutex
 }
 
 func NewBlock() *Block {

grid.go

@@ -153,6 +153,8 @@ func (self *Grid) Draw(buf *Buffer) {
 
 		entry.SetRect(x, y, x+w, y+h)
 
+		entry.Lock()
 		entry.Draw(buf)
+		entry.Unlock()
 	}
 }

render.go

@@ -6,6 +6,7 @@ package termui
 
 import (
 	"image"
+	"sync"
 
 	tb "github.com/nsf/termbox-go"
 )
@@ -14,12 +15,15 @@ type Drawable interface {
 	GetRect() image.Rectangle
 	SetRect(int, int, int, int)
 	Draw(*Buffer)
+	sync.Locker
 }
 
 func Render(items ...Drawable) {
 	for _, item := range items {
 		buf := NewBuffer(item.GetRect())
+		item.Lock()
 		item.Draw(buf)
+		item.Unlock()
 		for point, cell := range buf.CellMap {
 			if point.In(buf.Rectangle) {
 				tb.SetCell(