added file changes watching system

QFileSystemWatcher takes care of the changes in the file, and reloads
the project when a project is loaded if the user changes contents of the
file.
Just add a watch_file_changes parameter with value true to your
appleseed.studio.xml
Merge remote-tracking branch 'appleseedhq/appleseed@github/master'

Conflicts:
	src/appleseed.studio/mainwindow/mainwindow.h
small code tweaks.