fix project files definition (now excluding ignored/deleted files, and i...

...ncluding added/untracked files)

Problem with current ```get_project_files``` implementation is that it makes tasks like ```lint``` fail when version-controlled files have been deleted but not committed yet. Similar for newly added files.

I tried to slightly redefine what *project files* means, and came up with this solution.
Thus generated files (like PySide's resource files that have been mentioned in a comment) can be kept in ```.gitignore``` (which should be best practice anyway), and will be excluded from the project files list.