[UI][CORE] dragdrop deactivated in thumbnails,
fixed: crash in double click if location has no albums
[UI] fixed: if less than 5 pictures in folder, no thumbs are created
[UI][CORE] started implementation of album editing feature
[OTHER] updated screenshot
[OTHER] added doxygen comments and license header
[OTHER] added GNU/GPL license file
[OTHER] nothing relevant
[OTHER] added readme.md contents