fix: Enables Scene Builder to provide Insets editor for custom controls

[Oliver-Loeffler]

Enables editing of Insets properties in properties editor.
The MetadataIntrospector has been updated accordingly. The insets defaults are read from the introspected bean.

I'll add a test for this in a follow up commit. Ideally I'd like to move the functionality of makePropertyMetadata method into a separate class.

Issue

Fixes #532

Progress

• Change must not contain extraneous whitespace
• License header year is updated, if required
• The PR name must follow the pre-defined format
• Verify the contributor has signed Gluon Individual Contributor License Agreement (CLA)

[abhinayagarwal]

Overall the changes look good to me.

[Oliver-Loeffler]

Thanks @abhinayagarwal. I'wasnt fast enough for changing the license year. Well, but within next days I'll be able to review and rework the other open items.