Add paste support for UVs, materials, preferences, and undo

[karl-]

This PR introduces several improvements to the copy paste functionality in Unity:

• UVs are now imported and exported
• Materials now imported on paste (searches project for string matches)
• Meshes now share common vertices where possible (a preference is provided to disable this)
• Undo now works after a paste operation
• A preferences panel is now provided to toggle coordinate conversion and other settings