proctable: handle selection smartly when opening context menu

When the context menu is triggered by a mouse event, check the row
under the cursor: if it is not already part of the selection, select it
and deselect everything else.

This is the most common behavior, used for instance by Nautilus etc.

https://bugzilla.gnome.org/show_bug.cgi?id=705970
Handle the shift-multiselection case too like nautilus (bgo #705970)