Animation Editors: Channel Reordering

This commit restores the ability to reorder channels in the animation editors (DopeSheet/Action/etc., Graph/Drivers, NLA). The hotkeys for this are:
- Shift-PageUp = Move Up
- Shift-PageDown = Move Down

- Ctrl-Shift-PageUp = Move to Top of List
- Ctrl-Shift-PageDown = Move to Bottom of List

Do note that only animation data can get reordered using this. 
So, do not expect to be able to change the object order (that is actually taken from the order that Blender actually evaluates them per update/frame).

---

In the process, I've fixed a couple of other bugs:
* Removed 'optimisation step' check in anim_filter.c for dopesheet with no filtering options modified, since this meant that the ANIMFILTER_ANIMDATA data filter was not getting processed (and potentially there were other maintenance problems with that).

* Made NLA Editor's channel list not totally duplicate the basic Animation Channels keymap. Instead, the "NLA Channels" keymap now only defines the parts that are different, and this then gets specified before the standard one so that these different parts will override the standard ones.

* Attempted to fix BorderSelect on NLA Channels list. Still not totally working correctly yet though.

* Moved "Euler Discontinuity" menu entry from "Channels" to "Keys". It really belongs in the latter, since it affects the keyframe values, rather than some aspect of the channel (i.e. ordering of channels or how they're displayed)

release/scripts/ui/space_nla.py

@@ -122,6 +122,10 @@ class NLA_MT_edit(bpy.types.Menu):
         layout.operator("nla.move_up")
         layout.operator("nla.move_down")
 
+        # TODO: this really belongs more in a "channel" (or better, "track") menu
+        layout.separator()
+        layout.operator_menu_enum("anim.channels_move", "direction", text="Track Ordering...")
+
         layout.separator()
         # TODO: names of these tools for 'tweakmode' need changing?
         if scene.is_nla_tweakmode: