app: use material.Decorations on undecorated platforms

This patch implements a mechanism for customizing window
decorations.
If a window is configured with app.Decorated(true), then
the widget/material.Decorations are applied. On Wayland,
the option is automatically set when the server does not
provide window decorations.

Server side decorations are no longer requested.
The Decorated flag is set according to the
server's requests.

Wayland is now the default driver for UNIX platforms.

References: https://todo.sr.ht/~eliasnaur/gio/318
Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

app/os_android.go

@@ -1166,6 +1166,9 @@ func (w *window) Configure(options []Option) {
 		prev := w.config
 		cnf := w.config
 		cnf.apply(unit.Metric{}, options)
+		// Decorations are never disabled.
+		cnf.Decorated = true
+
 		if prev.Orientation != cnf.Orientation {
 			w.config.Orientation = cnf.Orientation
 			setOrientation(env, w.view, cnf.Orientation)
@@ -1188,12 +1191,17 @@ func (w *window) Configure(options []Option) {
 				w.config.Mode = Windowed
 			}
 		}
+		if cnf.Decorated != prev.Decorated {
+			w.config.Decorated = cnf.Decorated
+		}
 		if w.config != prev {
 			w.callbacks.Event(ConfigEvent{Config: w.config})
 		}
 	})
 }
 
+func (w *window) Perform(system.Action) {}
+
 func (w *window) Raise() {}
 
 func (w *window) SetCursor(name pointer.CursorName) {