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/Chat/RoomSidePane/MemberDelegate.qml

@@ -7,9 +7,12 @@ HTileDelegate {
     backgroundColor: theme.chat.roomSidePane.member.background
 
     image: HUserAvatar {
+        clientUserId: chatPage.userId
         userId: model.user_id
         displayName: model.display_name
-        avatarUrl: model.avatar_url
+        mxc: model.avatar_url
+        width: height
+        height: memberDelegate.height
     }
 
     title.text: model.display_name || model.user_id