Bugfix: Fix adding bookmarks on Gtk+ 3 based desktops

We used to add those bookmarks in a Gtk+ 2 compatible way only.
Now, bookmarks are added to the file belonging to Gtk+ 3, dropping support for end-of-life Gtk+ 2.
The bookmarks are now shown again for all Gtk+ 3 compatible file browsers, including Thunar, Nautilus, Nemo, Caja, etc.

https://github.com/owncloud/client/pull/9752
