Allow mod-directory-selection

[Krzmbrzl]

Once #132 and #133 is implemented this would be a further improvement:
Suppose I have a Modset in a subdirectory in my modfolder. Instead of having so select all Mods manually, it would be neat if I was able to just select the Modset-folder instead.

So basically the idea is to make folders selectable as well. And once a folder is selected all mods inside that folder (and subfolders) are automatically selected and loaded.