app: added support for fullscreen mode

The option field WindowMode allows changing the window mode of an application in either Windowed or Fullscreen.
Only macOS, Windows and X11 platforms are currently supported.

Updates gio#89.

Signed-off-by: pierre <pierre.curto@gmail.com>

app/internal/wm/window.go

@@ -19,8 +19,16 @@ type Options struct {
 	MinWidth, MinHeight unit.Value
 	MaxWidth, MaxHeight unit.Value
 	Title               string
+	WindowMode          WindowMode
 }
 
+type WindowMode uint8
+
+const (
+	Windowed WindowMode = iota
+	Fullscreen
+)
+
 type FrameEvent struct {
 	system.FrameEvent