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/editor.go

@@ -257,7 +257,7 @@ func (e *Editor) processPointer(gtx layout.Context) {
 					X: int(math.Round(float64(evt.Position.X))),
 					Y: int(math.Round(float64(evt.Position.Y))),
 				})
-				e.Focus(gtx)
+				gtx.Execute(key.FocusCmd{Tag: e})
 				if e.scroller.State() != gesture.StateFlinging {
 					e.scrollCaret = true
 				}
@@ -560,11 +560,6 @@ func (e *Editor) command(gtx layout.Context, k key.Event) {
 	}
 }
 
-// Focus requests the input focus for the Editor.
-func (e *Editor) Focus(gtx layout.Context) {
-	gtx.Execute(key.FocusCmd{Tag: e})
-}
-
 // Focused returns whether the editor is focused or not.
 func (e *Editor) Focused() bool {
 	return e.focused