Core: hot reload components from installed apworld (#3480)

* Core: hot reload components from installed apworld

* address PR reviews

`Launcher` widget members default to `None` so they can be defined in `build`

`Launcher._refresh_components` is not wrapped

loaded world goes into `world_sources` so we can check if it's already loaded.
(`WorldSource` can be ordered now without trying to compare `None` and `float`)
(don't load empty directories so we don't detect them as worlds)

* clarify that the installation is successful

typings/kivy/uix/boxlayout.pyi

@@ -0,0 +1,6 @@
+from typing import Literal
+from .layout import Layout
+
+
+class BoxLayout(Layout):
+    orientation: Literal['horizontal', 'vertical']

typings/kivy/uix/layout.pyi

@@ -1,8 +1,14 @@
-from typing import Any
+from typing import Any, Sequence
+
 from .widget import Widget
 
 
 class Layout(Widget):
+    @property
+    def children(self) -> Sequence[Widget]: ...
+
     def add_widget(self, widget: Widget) -> None: ...
 
+    def remove_widget(self, widget: Widget) -> None: ...
+
     def do_layout(self, *largs: Any, **kwargs: Any) -> None: ...