Tab switcher: Add TabswitcherFilesModel::raiseDocument() to avoid updating all items

Previously, raising a document (i.e. moving it to row 0) was done by
unregistering + registering a document again. This triggered several
calls of the expensive updateItems() call, even though updateItems()
is not needed at all in this case.

Now we directly raise via std::rotate(), following the nice talk
https://www.youtube.com/watch?v=W2tWOdzgXHA