Standalone material asset editor and gradient layer fixes

This update introduces a standalone material asset editor and resolves issues with gradient layer stop alpha on selection.

Changes

• New: Standalone material asset editor from the sidebar.
• Fixed gradient layer stop alpha on stop select.
• Fixed dodecahedron conversion with Smooth & Edit.
• Minor fix: the bottom part of the left sidebar occluding the layer.
• Minor fix: make tabs visible on electron when loading splash is active.