app: [API] refactor app.ViewEvent to support wayland

This commit changes the definition of app.ViewEvent on Unix systems
to be an interface implemented by two different concrete types:
X11ViewEvent and WaylandViewEvent. Each provides the necessary fields
for custom rendering on its display protocol.

This is a breaking change for all users of app.ViewEvent on
Linux/BSD, as it now is an interface instead of a concrete type.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

app/os_x11.go

@@ -813,10 +813,10 @@ func newX11Window(gioWin *callbacks, options []Option) error {
 		// make the window visible on the screen
 		C.XMapWindow(dpy, win)
 		w.Configure(options)
-		w.w.Event(ViewEvent{Display: unsafe.Pointer(dpy), Window: uintptr(win)})
+		w.w.Event(X11ViewEvent{Display: unsafe.Pointer(dpy), Window: uintptr(win)})
 		w.setStage(system.StageRunning)
 		w.loop()
-		w.w.Event(ViewEvent{})
+		w.w.Event(X11ViewEvent{})
 		w.destroy()
 	}()
 	return nil