widget: [API] replace Focus methods with explicit FocusCmds

Now that widgets by convention may be focused by issuing FocusCmd
directly, remove the now redundant Focus methods on Clickable, Editor,
Selectable.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/button.go

@@ -66,11 +66,6 @@ func (b *Clickable) Pressed() bool {
 	return b.click.Pressed()
 }
 
-// Focus requests the input focus for the element.
-func (b *Clickable) Focus(gtx layout.Context) {
-	gtx.Execute(key.FocusCmd{Tag: b})
-}
-
 // Focused reports whether b has focus.
 func (b *Clickable) Focused() bool {
 	return b.focused