Use the current Action API (i.e. move away from the to-be-deleted-in-5.0
one) to import and export F-Curves from/to FBX files.
There is a slight difference in functionality for the exporter, in the
selection of which Actions to export for the "All Actions" option. This
is just a minimal change to ensure the legacy API is no longer used.
Old: `action.fcurves` was iterated, and if all FCurves could resolve to
existing properties, the Action was exported. This would only work
reliably for single-slotted Actions, due to the use of the deprecated
`action.fcurves` property.
New: the above check is done for each Channelbag in the Action. The
first Channelbag that match the above check is exported. This does _not_
export all suitable channelbags; it merely improves on the old behaviour
slightly. The new, C++-based FBX exporter is more feature-complete in
this regard.
This is part of #146586
8536cd794e