Make the resource type's default theme item editable in Theme editor

[Rindbee]

In most cases, we may only need to modify some properties of the default resource instead of all. But click the plus icon to override item, it will use an empty object to override. Allow EditorResourcePicker to be editable, you can use the Make Unique option to create a copy and then edit it.

001.mp4

[YeldhamDev]

• Clearing a default item and then undoing it doesn't set it back.
• Making a default item unique and undoing it makes the inspector show the property as empty.
• Not a regression from this PR, but the "Edit" button is available and does nothing when clicked. It would be nice if it behaved like the "Make Unique" option, if possible.

[Rindbee]

• Clearing a default item and then undoing it doesn't set it back.

1.mp4

• Making a default item unique and undoing it makes the inspector show the property as empty.

2.mp4

• Not a regression from this PR, but the "Edit" button is available and does nothing when clicked. It would be nice if it behaved like the "Make Unique" option, if possible.

Perhaps it's because the property doesn't actually exist. This issue also occurs within the control's theme overrides.

Fixed in #117659.

2.mp4

[akien-mga]

Thanks!