Project manager

I am back haha, recently having work on my master thesis.
This PR and branch are gonna clean up qucs main program, there will be some further work on this branch. However, I think that too large PR will be more difficult to review and merge.
The PR now is refactoring project viewer at dock window with listview and QFileSystemModel. The model is always sync with file system state, so we don't need call "refresh" every time.
I also examine the code in qucs.cpp, and giving it some clean up.