document-manager: bgo#698036 - Close Other Tabs enhancement

Added a "Close Others" option to the notebook's popup menu. The menu requires to be generated manually because of GtkNotebook's restrictions; doing this in anjuta_docman_create_tab_popup function inside anjuta-docman.c. The menuitems are connected to on_tab_popup_click and on_close_other_file_activate_from_popup. The menu is destroyed if idle using gtk_widget_destroy.