Add reflected Map controls

[B-Reif]

This PR adds support for editing reflected Map values. Specifically:

• mutable fields support for editing Map values in-place
• adds 'remove element' buttons for each key-value pair
• adds a 'draft element' field to edit and add new key-value pairs

It should be noted the 'add element' draft implementation is pretty inefficient. It uses egui's IdTypeMap which internally clones the element, and also clones the element itself to move the value out of state for use. If we have a better way to store this (on the InspectorUi struct? in the Bevy world?) that would be an improvement.

[jakobhellermann]

Very nice, thanks you!

It should be noted the 'add element' draft implementation is pretty inefficient. It uses egui's IdTypeMap which internally clones the element, and also clones the element itself to move the value out of state for use. If we have a better way to store this (on the InspectorUi struct? in the Bevy world?) that would be an improvement.

I think for an a feature like this it doesn't matter too much if this does some unnecessary clones. When it becomes obviously slow we can reconsider.