Blender Git Loki

Dopesheet: Added "Moving Hold" as a keyframe type

Currently "long keyframes" are only useful for indicating where stationary
holds occur. If however you try to create a "moving hold" (where the values
are slightly different, but in terms of overall effect, it's still a hold)
then it could get tricky to keep track of where these occur.

Now it's possible to tag such keyframes (using the keyframe types - RKEY)
as being part of a moving hold. These will not only be drawn differently
from normal keyframes, but they will also result in a "long keyframe"
being drawn between each pair of them, just like if they had been completely
stationary instead.

Currently the theming/styling of these is a bit rough. They reuse the existing
theme colours for long keyframes.