widget,widget/material: make Clickable widgets focusable

This change adds focus and keyboard control to Clickable widgets.
They now consider a press of the enter or return key equivalent to
a click. To keep the change simple, the focus indication is the
same as the hover indication.

References: https://todo.sr.ht/~eliasnaur/gio/195
References: https://github.com/tailscale/tailscale/issues/1611
Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/bool.go

@@ -23,16 +23,21 @@ func (b *Bool) Changed() bool {
 	return changed
 }
 
-// Hovered returns whether pointer is over the element.
+// Hovered reports whether pointer is over the element.
 func (b *Bool) Hovered() bool {
 	return b.clk.Hovered()
 }
 
-// Pressed returns whether pointer is pressing the element.
+// Pressed reports whether pointer is pressing the element.
 func (b *Bool) Pressed() bool {
 	return b.clk.Pressed()
 }
 
+// Focused reports whether b has focus.
+func (b *Bool) Focused() bool {
+	return b.clk.Focused()
+}
+
 func (b *Bool) History() []Press {
 	return b.clk.History()
 }