Form Variation inspector polish

[youarebritish]

Make the Form Variation inspector more user-friendly.

Some example enhancements:

• Separate out the different settings into tabs instead of showing them all at once
• Instead of manually entering strings/hashes, have a Unity Object selector
• Add an "Attach to Model" button which attaches the Form Variation to a FoxModel, allowing you to select from its mesh groups, etc from a drop-down

[Joey35233]

See https://github.com/youarebritish/FoxKit/issues/1