Issue #2303 - Please add Constant type of gradient interpolation ...

... to make multi-color hard-edge gradient fills possible

Add a new "step" gradient-segment blending function, which is 0
before the midpoint, and 1 at, and after, the midpoint.  This
creates a hard-edge transition between the two adjacent color stops
at the midpoint.  Creating such a transition was already possible,
but required duplicating the same color at the opposing ends of two
adjacent stops, which is cumbersome.

(cherry picked from commit 68bf99e806)

menus/gradient-editor-menu.xml

@@ -85,6 +85,7 @@
       <menuitem action="gradient-editor-blending-sine" />
       <menuitem action="gradient-editor-blending-sphere-increasing" />
       <menuitem action="gradient-editor-blending-sphere-decreasing" />
+      <menuitem action="gradient-editor-blending-step" />
       <menuitem action="gradient-editor-blending-varies" />
     </menu>
     <menu action="gradient-editor-coloring-type">