fix: Reduce visual noise when loading in new properties in the property grid

[Eideren]

PR Details

The property grid is briefly emptied and categories are expanded after a short delay whenever the user swaps between two selected entities:

Stride.Gamestudio.Bf98qp7j86.mp4

This PR ensures the previous one is kept on screen while the other one loads in:

Stride.Gamestudio.Sywj9dwutx.mp4

This improves readability when comparing two objects, or when quickly scanning for a given property or component.

Loading duration has been artificially lengthened in the two clips above for clarity.

This area probably has a bunch of conflicts with your branch @Kryptos-FR , do let me know if I should leave this PR for later! Also I have zero clue about WPF, probably did something wrong, let me know ;)

Related Issue

None afaict

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.

[Kryptos-FR]

I'm a bit sick at the moment so I can't build locally. Let me ask a few questions instead.

Did you check if it had any impacts when selecting entities either from the tree, or picking from the scene? Or with multiple selection as well as "navigating" back and forth with those selection?

Some of the previous code had checks to prevent reentries but I don't remember what scenarii were triggering it