'open' option for subfolders, configurable menus, vec2.unit of null vectors.

-> Added 'open' option to subfolders in projectTree widget, so PNs are displayed when created.
-> Added dictionaries to configure different menues and a function to build them.
-> Changed the unit property to return a null vector when the original vector is null, because most of the times, the idea is to perform some vector arithmetic and this way it would work.