Blender Git Loki

UI: Ctrl-click on panel header expands & collapses subpanels

If the panel is already open and you ctrl click on its header, its
sub-panels are now expanded / collapsed instead of toggling the
expansion for all other panels. If you're toggling a panel's expansion
it's already in view anyway, so there is no need to collapse the
other panels.

Differential Revision: https://developer.blender.org/D8042