Blender Git Loki

This commit adds some missing features to markers in action editor,
and also fixes some further segfaults.

Now it is possible to 'surf' (jump) to different markers. This is only
possible through the hotkeys for this:
* Page Up - Next Marker
* Page Down - Previous Marker

These hotkeys match the ones in the timeline. In order to do so,
I've had to reassign two (rarely used/obscure) hotkeys:
* 'Ctrl Page Up' is now move selected action channel up (was Page Up)
* 'Ctrl Page Down' is no move selected action channel down (was Page Down)
I should also mention here that
* 'Shift Page Up' is move action channel to top
* 'Shift Page Down' is move action channel to bottom