Enable proper project handling

As a designer I need to be able handle overall presentation PROJECT that has many presentations. This ability was lost when stripping the original architect component away from Qt 3D Studio releases.

As a designer I need to often create sub-presentations for both 3D and 2D content, that are then streamed as textures in to the parent presentation. The workflow at the moment for doing this is highly convoluted to the point where we can almost say we don't have support for sub-presentations at all in the editor.

As a designer I need to be able to also open sub-presentations for editing side by side in the editor to enable smooth workflow for sub-presentations. As a user I need to see what presentations are in this project and be able to open those from a project view.

To fix this the editor must be able to read and edit the UIA project file, show the sub-presentations visually as part of the project and allow user to easily create both 2D and 3D sub-presentations with context menus etc.

See original suggestion QT3DS-262 for sketches how the UI could work.

As technical requirement, the whole idea "what is a Qt 3D Studio project" needs to be thought about. Today .uia file acts as a sort of project file, but is it really needed? Can project be just a collection of directories that are scanned and the project model is then built from that? Does this cause problems for runtime performance if we do this?

A wish is to loose the .uia file completely and move to using the directory structure as project. But this is a wish, not a strict requirement.