Code refactoring and SVG import improvements

This update includes significant code refactoring and improvements to SVG import functionality.

Changes

• We refactored a bunch of the code! (about 60% of it).
• A lot of files importing bug fixing.
• SVG import now works a lot better (e.g., you can export Figma/Sketch designs as SVG and import on Spline).
• Imported textures (JPG/PNG) now appear on the texture palette so they can be reused.
• Fixed a lot of bugs related to materials.