Refactoring the UI classes with .ui files

Hello,

I wanted to work on #12, but it's practically impossible do it in .cpp files...
So, I was wondering if refactoring the dialogs to use qt designer (.ui) files would be something acceptable ? It makes the code cleaner and more readable IMO.
Back to my point, using the designer makes UI integration (using CSS styling) waaaaay easier :)

I started this refactoring on a separate branch, it doesn't seem to be a job too difficult, is it something you would be interested in ? I created this pull request to have your opinion on it.