[cosmetic] use first artwork image instead of last for set thumbnail in the tree view (closes #17)
[cosmetic] properly align images vertically in an image list
[cosmetic] display groups in a set into a light blue cell, like in the tree (closes #18), and btw improved CSS handling a bit (unfinished)
[evol] added artwork view and now use the new image frame in all views
[evol] added group view and linked it from the tree and set views
[evol] added set view and linked it from tree view
[fix] sets should be ordered in tree view
[evol] created display_artwork_frame() helper function and now use it in the welcome page + created a set/group tree view with it too