Update file open/save dialogs

#### Issue #90 ####
* Locations Desktop and Downloads have been added. If you think another location should be added too, let me know.
* For this, introduced a new subclass of QFileDialog in order to prevent code duplication.

#### Issue #86 ####
* Added new subclass VersionedSaveDialog.
* Use this dialog for saving project files
* Added "Save as new version" to project menu