Blender Git Loki

Context: add "active_nla_track" & "active_nla_strip" context members

Selection was already accessible but not active.

Add utility functions:

- ANIM_nla_context_track to access the active track,
following the convention of ANIM_nla_context_strip.

- ANIM_nla_context_*_ptr versions of these functions,
needed to for creating context members to access the ID pointer.

Part of fix for T90723.