Dual-screen & macOS fixes

Improvements for all users:

• All dual-screen window positioning issues should be fixed.

Improvements for macOS users:

• Notifications used to be quite broken on macOS. Everything should just work
  now.
• Special Icon\r files were not properly ignored during synchronization,
  probably due to a weird bug in some third-party component. Although it could
  make sense to synchronize them between 2 macOS computers, the cozy-stack
  actually doesn't support creating a file with name containing a \r
  character. We made sure those files were properly ignored.

Improvements for developers:

• Test reports recently stopped working when run with coverage enabled (e.g. on
  CI). Everything is back to normal.

See also known issues.

Happy syncing!