Rewrite media caching (old image provider)

- Doesn't use pyotherside's image provider feature, for more flexibility
  and simplicity
- Suitable for supporting matrix media events and more later
- Avoid a lot of duplicate files that the old cache created due to
  server not returning what we expect, mistakes in Python/QML code, etc
- Changed file structure
  (e.g. thumbnails/32x32/<mxc id> instead of
   thumbnails/<mxc id>.32.32.crop)

- Backend.wait_until_account_exist: start issuing warnings if the
  function runs for more than 10s, which means in most case a bad user
  ID was passed

- New HMxcImage QML component, used in H(User/Room)Avatar

src/python/matrix_client.py

@@ -93,6 +93,10 @@ class MatrixClient(nio.AsyncClient):
 
         self.skipped_events: DefaultDict[str, int] = DefaultDict(lambda: 0)
 
+        from .media_cache import MediaCache
+        cache_dir        = Path(self.backend.app.appdirs.user_cache_dir)
+        self.media_cache = MediaCache(self, cache_dir)
+
         self.connect_callbacks()