invalidatable: cleanup dead code

Remove invalidatable type and all associated calls. All items can
directly invalidate the UI.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/ui/popover.go

@@ -53,11 +53,5 @@ func (p *Popover) Focus(f bool) {
 }
 
 func (p *Popover) Invalidate() {
-	p.content.Invalidate()
-}
-
-func (p *Popover) OnInvalidate(f func(Drawable)) {
-	p.content.OnInvalidate(func(_ Drawable) {
-		f(p)
-	})
+	Invalidate()
 }