- 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
