Improve sub-presentation handling

Today Qt 3D Studio handles only single presentation editing at a time. This is not sufficient as there is clear need to be able to open a whole 3D Studio project (.uia file) and be able to:

• Browse the presentations in that project file.
• Create new subpresentations easily with simple right click action.
• Delete subpresentations from the project.
• Open multiple presentations for editing simultaneously with quick switching between those.
• Share objects between sub-presentations (no need to load same object multiple times).