Blender Git Loki

Cleanup: calculating sample frames get scene action instead of object

Previously the functions for calculating the sample frames have used
the animated object from where they derived the active action. This was
not necessary. I changed the api so that the scene action is now
directly specified for better reading.

Cleanup: Added Helper functions to get Scene actions

This is mostly for cleaning up the AnimationExporter for
supporting the upcoming feature "export all animations".
I also added a few extra checks for existing animations,
which potentially avoid crashes when the calling functions
do not check for existing animations.

Removed const qualifier from EvaluationContext parameter and adjusted the class methods to reduce future merge conflicts in Blender2.8