Make sure to not load tooltip images unless the user is
currently trying to see one by hovering on an avatar.
This also seems to reduce idle CPU usage.
thumbnail() and download() were remaining.
Use a direct and carefully chosen MatrixClient's methods instead to
avoid problems mentioned in the previous commit 7502c1.
Fix bug introduced in 11fb32:
When loading an encrypted thumbnail, QML lacks the decryption dict for
half a second at first.
When calling python like this, python calls the wrong matrix API
for fetching the encrypted thumbnail, and the added retry code
would be forever stuck.
The retry code has been moved to QML, and now works for all HImage.
