[WIP] refactor(v2.0.0): Restructure UI + core improvements

[tacf]

Purpose of this PR:

• Extract and generalize the implicit data structures (like binary tree used in rootview)
• Create base drawing generic utils for re-use (like centered text, labels/quad with text)
• Setup builder like pattern model to simplify UI setup (maybe?)

Accomplished:

• Engine owned title bar rendering + simulated window shadow (lost the rest of the hair on this one)
• Tree structure extracted and rootview now uses it instead of it's own b-tree implementation for views (to be seen if this will be re-usable)
• Moved some files around for better file structure (hopefully)
• SDL3 took some GPL-1 and is now statically linked so we ship only a single binary