Tracking: Expose track average in the menu

Patch by Sebastian Koenig, thanks!

Tracking: Fix wrong interpolation at the end of markers array

Tracking: Implement tracks average operator

Average selected tracks into the new one. This can be used to improve
stability of tracking on blurry or non-very-sharp feature shapes.

Averaging happens for all position, pattern corners and search area.
Disabled markers do not take effect on averaging. Keyframed flag is
copied from source.

Gaps in the source tracks will be linearly interpolated, to reduce
result track jump. Note that this only applies to gaps "inbetween".
This means that if an input track doesn't have markers in the
beginning/end of it, there is nothing to interpolate with and the
result track will jump.

Available from the Track panel, under the Merge category.

Differential Revision: https://developer.blender.org/D6323

Tracking: Add interpolated accessor for markers

Allows to get marker position interpolated through out a gap.

Currently unused, but is covered with test. Aimed to be used for
track average operator.