Expose delete-section function

(insert-section still remains hidden, it inserts the length
of the selection at the edit-point, it should perhaps insert
at the start of the selection instead)

gtk2_ardour/ardour.menus.in

@@ -201,6 +201,7 @@
       <separator/>
       <menuitem action='cut-paste-section'/>
       <menuitem action='copy-paste-section'/>
+      <menuitem action='delete-section'/>
       <separator/>
       <menu action="SelectMenu">
         <menuitem action='select-all-objects'/>