Sketch: add command to context-menu to change value of constraint

The edit datum dialog is opened by double-clicking on a dimensional constraint in a sketch. However, the double-clicking event doesn't work realiably on
some systems. As a workaround this PR adds the command to the context-menu.

For more details see the forum thread: https://forum.freecad.org/viewtopic.php?t=71137

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -4142,6 +4142,9 @@ void ViewProviderSketch::generateContextMenu()
 
         // context menu if only constraints are selected
         else if (selectedConstraints >= 1) {
+            if (selectedConstraints == 1) {
+                menu << "Sketcher_ChangeDimensionConstraint";
+            }
             menu << "Sketcher_ToggleDrivingConstraint"
                  << "Sketcher_ToggleActiveConstraint"
                  << "Sketcher_SelectElementsAssociatedWithConstraints"