Add popovers

A popover is a special UI element which can be layered over the rest of
the UI (i.e. it is painted last) and can fall anywhere on the screen,
not just with the bounds of its parent's viewport/context. With these
special abilities comes the restriction that only one popover may be
visible on screen at once.

Popovers are requested from the UI context passed to Draw calls and
specify the anchor point and the desired dimensions. The popover is then
fit to the available space and placed relative to the anchor point.

lib/ui/context.go

@@ -10,9 +10,10 @@ import (
 
 // A context allows you to draw in a sub-region of the terminal
 type Context struct {
-	screen   tcell.Screen
-	viewport *views.ViewPort
-	x, y     int
+	screen    tcell.Screen
+	viewport  *views.ViewPort
+	x, y      int
+	onPopover func(*Popover)
 }
 
 func (ctx *Context) X() int {
@@ -35,9 +36,9 @@ func (ctx *Context) Height() int {
 	return height
 }
 
-func NewContext(width, height int, screen tcell.Screen) *Context {
+func NewContext(width, height int, screen tcell.Screen, p func(*Popover)) *Context {
 	vp := views.NewViewPort(screen, 0, 0, width, height)
-	return &Context{screen, vp, 0, 0}
+	return &Context{screen, vp, 0, 0, p}
 }
 
 func (ctx *Context) Subcontext(x, y, width, height int) *Context {
@@ -49,7 +50,7 @@ func (ctx *Context) Subcontext(x, y, width, height int) *Context {
 		panic(fmt.Errorf("Attempted to create context larger than parent"))
 	}
 	vp := views.NewViewPort(ctx.viewport, x, y, width, height)
-	return &Context{ctx.screen, vp, ctx.x + x, ctx.y + y}
+	return &Context{ctx.screen, vp, ctx.x + x, ctx.y + y, ctx.onPopover}
 }
 
 func (ctx *Context) SetCell(x, y int, ch rune, style tcell.Style) {
@@ -113,3 +114,13 @@ func (ctx *Context) SetCursor(x, y int) {
 func (ctx *Context) HideCursor() {
 	ctx.screen.HideCursor()
 }
+
+func (ctx *Context) Popover(x, y, width, height int, d Drawable) {
+	ctx.onPopover(&Popover{
+		x:       ctx.x + x,
+		y:       ctx.y + y,
+		width:   width,
+		height:  height,
+		content: d,
+	})
+}