This allows gizmo groups to store properties in the tool.
This makes sense for gizmo options which only control gizmo display and
don't control operator execution.
Unlike similar kinds of properties,
this isn't accessible via the gizmo-group-type instance.
For now the it's only stored in the workspace tool as can be done for
operator properties, so each instance doesn't have different settings
which would be confusing from a user perspective and complicate access
from the top-bar.
Later we could add gizmo-group properties if needed.
It was a bit odd that the scene was stored per window but not the view
layer. The reasoning was that you would use different view layers for
different tasks. This is still possible, but it's more predictable to
switch them both explicitly, and with child window support manually
syncing the view layers between multiple windows is no longer needed
as often.
