Blender Git Loki

NLA Editor Drawing: Set up AnimChannelType callbacks for drawing NLA Editor

This commit lays some of the groundwork needed to port the last of the NLA Editor
channel types that's still defined using custom code. By moving this code over
to the newer standardised framework, this will enable widgets to have tooltips
to make it easier for users to find out what various buttons do.

Currently this isn't hooked up to anything though.