Add ability to change bookmark's name directly from roster

Description of the new feature

Gajim should allow user to change bookmark settings (name) directly from a roster. This should be done in the same way like we now can change people name. When user click right mouse button on selected bookmark item, Gajim should show context menu with additional option - "Change bookmark". When user click this field, Gajim should show a window "Group chat bookmarks" and focus list on this bookmark.