cli (call_gui): Fix Qt events processing:

`app.sendPostedEvents` was not handling all events (notably not the window system ones),
resulting in side effects such as the image freezing during A/V calls when the mouse was
not moving for a few seconds.

Instead, `app.processEvents` is now used, even though the documentation discourages its
use: in this context, it should be fine, and it's a simple solution.

libervia/cli/call_gui.py

@@ -198,8 +198,8 @@ class AVCallUI(QMainWindow):
     @staticmethod
     async def run_qt_loop(app):
         while running:
-            app.sendPostedEvents()
-            await asyncio.sleep(0.1)
+            app.processEvents()
+            await asyncio.sleep(0.01)
 
     @classmethod
     async def run(cls, parent, call_data):