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/qml/SidePane/AccountDelegate.qml

@@ -46,9 +46,10 @@ HTileDelegate {
 
 
     image: HUserAvatar {
+        clientUserId: model.data.user_id
         userId: model.data.user_id
         displayName: model.data.display_name
-        avatarUrl: model.data.avatar_url
+        mxc: model.data.avatar_url
     }
 
     title.color: theme.sidePane.account.name

src/qml/SidePane/RoomDelegate.qml

@@ -32,8 +32,9 @@ HTileDelegate {
 
 
     image: HRoomAvatar {
+        clientUserId: model.user_id
         displayName: model.data.display_name
-        avatarUrl: model.data.avatar_url
+        mxc: model.data.avatar_url
     }
 
     title.color: theme.sidePane.room.name