Blender Git Loki

Small animation tweaks:
- Fixed problem where just trying to replace existing keyframes would
result in the intepolation set on that keyframe to get lost. This was
mostly an issue if trying to re-block some animation in the middle of
a shot, with the rest of the keys set to Bezier, but the first
keyframe in this new segment needing to be Constant so that we don't
get sloppy automatic interpolation in the way

- Hooked up Media-Play/Stop/Next/Prev controls to animation playback
and keyframe jumping functionality in default keymap in addition the
existing controls. I'm also considering whether to migrate Next/Prev
Keyframe key mappings off the Ctrl-PageUp/Down keys for a more
ergonomic option (i.e. shift <, shift >)