Draft: New launcher

In this branch, some modifications have been done to support the new launcher:

• The QtWidgets-based launcher has been removed (SolarusGui and the gui directory have been removed) because the new launcher uses QtQuick.
  • NB: The removed QtWidgets files should be instead moved to the quest editor repository (done in this MR).
• New quest properties have been added:
  • languages (list)
  • initial_release_date (date as string, YYYMMDD)
  • license (string)
  • min_players (integer)
  • max_players (integer)
  • genre (string)