Save expansion states of metadata groups in editor

[solth]

Whenever the metadata table in the metadata editor is updated, all metadata groups are expanded every time. This can become very annoying when there are multiple levels of metadata groups that the user wants to collapse to maintain a better overview over the metadata table.

Solution: The metadata groups should remember their expansion state and keep it until the user explicitley changes the expansion state or until the the metadata editor is closed and re-opened again, similar to how expansion states are saved in the structure trees.