[Download] Added fallback to snapshot version

The downloader will accept a snapshot version in case no release version is to be located.
[Download] Improved directory selection

 - Fixed some errors in selection pages
 - Don't accept binary data directories that are not labeled as such or
   are empty
 - Fixed crash in case the directory selection is canceled
[Common] Directory marker for all directories
[Downloader] Development snapshot of the cleaning component
Merge remote-tracking branch 'GitHub-Vilarion/feature/gradle' into feature/gradle
[Downloader] Add layout and texts for uninstall window