Add support for keyframe control

[mwestphal]

Is your feature request related to a problem? Please describe.
While F3D support playing animation, there is no way to go to the next/previous keyframe

It would be nice to be able to do that

Describe the solution you'd like

• Add support in all importers to recover keyframes values (or timestep in VTK linguo) (see below for status)
• Add interactor::goToKeyframe(size_t index, bool relative)
• Add command go_to_next_keyframe, got_to_previous_keyframe using above API

Additional context
Status for timesteps support:

• vtkF3DAssimpImporter: TODO
• vtkGLTFImporter: Small fix TODO
• vtkF3DQuakeMDLImporter
• vtkF3DUSDImporter: TODO
• vtkF3DGenericImporter: TODO
• vtkF3DMetaImporter: TODO