Separate action for editing fields model XML

[tlotze]

In a customer project, we had the situation that some field was misconfigured in such a way that the fields view wouldn't load anymore and thus, the field couldn't be removed either through the UI or by editing the XML. Using export/import might have been a possibility but the XML data is encoded upon export in a way that isn't easily editable.

I suggest an action for editing the XML, either in addition to or instead of having to go through the fields form in order to edit the XML. Such an action would allow resolving a catch-22 situation like described above.