VAB Save-Load / Workspace improvements

[Vl3d]

I think we really need this ASAP because backward compatibility will become an issue later. It's a very important change and relatively easy to implement:

• In the Save / Load windows, replace the term "Vehicle" with "Project" (because some people build entire families / fleets of boosters as assemblies and these are not vehicles)
• Also, replace the term "Workspace file" with "Project workspace snapshot"
• In the Load window, replace "Load vehicle" with "Load project" AND put all "Project workspace snapshots" in a drop-down list specific for each Project save.

This way we can have Projects be vehicles and then we can merge / import that vehicle Project Workspace Snapshot into another Project which is a fleet of vehicles (but we should be able to name these vehicles individually). 
Also, as stated above, it would be incredibly useful to be able to label (sub)assemblies and have them be a single group of parts which can be stacked and destacked. More details below:

• A Project should be a collection of vessels and assemblies (like a folder which holds crafts and (sub)assemblies).
• We should be able to label vessels and (sub)assemblies (groups of parts) inside the Project. 
• Labeled craft / (sub)assemblies (groups of parts) should also save their own action groups and staging sequences.
• The assemblies should stay grouped together even if stacked on top of each other, so it's easy to destack them (stage, lander, return pod etc.). Working on parts of an assembly would require selecting that group.

Edited March 1, 2023 by Vl3d