widget: [API] change Decorations to leave the user in control of window state

As suggested by ~egonelbre, Decorations should not be the source of truth
for the windows state, because external gestures may also change state.

This breaking change removes Decorations.Perform and exposes Maximized
as a bool which is the user's responsibility to set.

Fixes: https://todo.sr.ht/~eliasnaur/gio/600
Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/material/decorations.go

@@ -75,7 +75,7 @@ func (d DecorationsStyle) layoutDecorations(gtx layout.Context) layout.Dimension
 				case system.ActionMinimize:
 					w = minimizeWindow
 				case system.ActionMaximize:
-					if d.Decorations.Maximized() {
+					if d.Decorations.Maximized {
 						w = maximizedWindow
 					} else {
 						w = maximizeWindow