Beta Release 1.15.15

The most visible change added with this release is that when a major version is newly installed Wesnoth will migrate user preferences and attempt to redownload add-ons from its new add-ons server.

• Updated translations: British English, Bulgarian, Chinese (Traditional), Czech, Italian, Portuguese (Brazil), Russian, Spanish, Turkish
  

• Added a prompt to allow migrating settings and redownloading add-ons used in a previous version of Wesnoth when starting a new versions for the first time.
  

• Fixed: keyboard input during the loading screen intermittently caused a crash (issue #4447)
  
• AI: fixed custom synced commands not changing the game state