Context Menu gives the option to mark any location as favorite with the user given name.
ZoomDialog also takes description as parameter, to be used for favorite option also.
Favorites which are added through newly added context menu option don't
get loaded in next session of maps.
Comment the part which checks for id attribute of place before loading favorites in
next section. 'id' attribute doesn't get used anywhere.

closes GNOME/gnome-maps#144