Core: Launcher: can drag-and-drop patch on Launcher window (#3442)

* Core: Launcher: can drag-and-drop patch on Launcher window

* doc string for `_on_drop_file`

typings/kivy/core/window.pyi

@@ -0,0 +1,15 @@
+from typing import Callable, ClassVar
+
+from kivy.event import EventDispatcher
+
+
+class WindowBase(EventDispatcher):
+    width: ClassVar[int]  # readonly AliasProperty
+    height: ClassVar[int]  # readonly AliasProperty
+
+    @staticmethod
+    def bind(**kwargs: Callable[..., None]) -> None: ...
+
+
+class Window(WindowBase):
+    ...

typings/kivy/event.pyi

@@ -0,0 +1,2 @@
+class EventDispatcher:
+    ...