Automatic updates

This is for testing purpose only.
Do not install it, unless you know what you're doing.

Improvements for all platforms:

• A new auto-updates system which should work better on both Windows and macOS
• Fixed an issue preventing a locally renamed file to be updated on the remote
  Cozy

Improvements for Windows users:

• The weird animation that was displayed while installing the app since the
  alpha4 release should be replaced with a normal install dialog.

Finally, we would like to apologize to our Windows users, since auto-updates
actually didn't work in the alpha4 release. This should be fixed now. Thanks
for your patience.

Happy syncing!