Fixes: #36 #37 #38 #40 #41 + Subpackages page works

1. Small stars near labels on import page - I think they look pretty.
2. User won't be switched to next page until he'll provide path to existing file or directory, if doesn't exist - on click "Next button" page will not be switched and import filed will be highlighted (looks better now)
         + same highlighting works while user typing path (-> pathlib used).
3. Subpackages works now (files from left side moves to the tree of sub-packages) - only thing I'll need a list of files that could be added to sub-packages.
4. About importing directory  - right now I can't find cool way to work it out with one button (so I came up with those two - another way was radio buttons, but later I'll figure out how to do it with one button) - just there is no standard function for doing this.