The existing Add and Multiply blending modes have limited usability,
because the appropriate operation for meaningfully combining values
depends on the channel: e.g. add for location, multiply for scale,
and quaternion multiplication for rotations.
This adds a new mode that chooses the operation automatically based
on property settings. Strips with this mode can also be keyframed
normally in Tweak mode, except that the channel value sliders on the
left side of Graph/Action editors won't insert keys without Auto Key.
Supporting a strip blending type that treats quaternions as a unit
also means being able to adjust all sub-channels as a unit when
inserting keyframes. This requires refactoring keyframe insertion
code to retrieve array property values for all channels at once,
before iterating over the indices being inserted.