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>

app/ime_test.go

@@ -34,7 +34,7 @@ func FuzzIME(f *testing.F) {
 
 		var r input.Router
 		gtx := layout.Context{Ops: new(op.Ops), Source: r.Source()}
-		e.Focus(gtx)
+		gtx.Execute(key.FocusCmd{Tag: e})
 		// Layout once to register focus.
 		e.Layout(gtx, cache, font.Font{}, unit.Sp(10), op.CallOp{}, op.CallOp{})
 		r.Frame(gtx.Ops)