Launcher: do not automatically check remote repositories

 - instead, add a button to the launcher which allows manual downloading
 - add a configuration option autoCheckRepositories to get the old
   behaviour

This feature disables the automatic check on every startup of the vcmilauncher but allows to get back the old behaviour by setting the configuration variable autoCheckRepositories to true.

This feature is important for Debian because no program should access a remote server without consent of the user. I imagine that a user might not be aware that vcmilauncher will automatically contact download.vcmi.eu until they run the program and see the progress bar (at which point it is too late). Also in its current form, vcmilauncher forces contacting download.vcmi.eu on every program start-up. So right now, it is impossible to start the launcher without informing the server behind download.vcmi.eu that you are now gonna play the game.

At the very least, please add this patch but set the default value of autoCheckRepositories to true. That way, I can easily set the value to false for the Debian packaging. This would already help a lot, thanks!