Propagate Pose: Added 'Selected Keyframes' mode

This commit adds a new mode for the Propagate Pose tool. With this new option,
the Propagate Pose will copy the current pose over to all selected keyframes
after the current frame.

For reference, some of the other/existing options are: to copy it to each subsequent
keyframe with the same value (WHILE_HELD - the default), to the next keyframe,
or to the last keyframe.

release/scripts/startup/bl_ui/space_view3d.py

@@ -1933,6 +1933,10 @@ class VIEW3D_MT_pose_propagate(Menu):
 
         layout.separator()
 
+        layout.operator("pose.propagate", text="On Selected Keyframes").mode = 'SELECTED_KEYS'
+
+        layout.separator()
+
         layout.operator("pose.propagate", text="On Selected Markers").mode = 'SELECTED_MARKERS'