Blender Git Loki

UI: NLA Editor Sidebar overhaul

The NLA Editor sidebar is currently out of step with the rest of Blender, and in particular the Sequencer, which offers similar features and functionality. This patch makes a number of UI layout changes to make it fit in.

- Remove Active Track panel, since the track names can just be edited in the source list on the left
- Rejiggered the categories to be more understandable. 'Strip' includes settings for the current strip, and 'Edited Action' includes settings for the action that is currently being edited
- Use single column split layout
- Use name + mute toggle configuration like in the Sequencer
- Fix an issue with the theme, where the sidebar background was too dark, breaking the bright -> dark hierarchical logic
- Use sub-panels for animated strip properties controls

Differential Revision: https://developer.blender.org/D6687

Reviewed by Brecht van Lommel