Beta update, path/name limits, running app detection & Windows localization

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

Improvements for all platforms:

• A beta release! Although some cases remain where synchronization breaks, it
  mostly works on all 3 platforms. Please report every issue!
• Switching to beta was good timing to change the version number format, which
  should finally fix the last auto-updates issue.
• The app will detect and prevent synchronization of files/folders with
  paths/names too long for your system, so you never end up with unusable
  content on your hard drive.
• When starting Cozy Desktop, it will detect an already running instance of the
  app, make it visible and exit (instead of displaying a weird error message).

Improvements for Windows users:

• The app now detects and displays using the user language.

Happy syncing!