Currently only available through search menu. Need to have an input
from VFX team to see where button will fit best. So to test press F3
to access search and type "average". Should bring to "Average Tracks"
The goes as following: all selected tracks are averaged into a new
one, which becomes active the operator is finished. This allows to
preserve input for possible future tweaks.
Disabled markers do not take effect on averaging.
Keyframed flag is copied from source.
Averaging happens for all position, pattern corners and search area.
Pattern and search area averaging is a bit disputable, but decision
here will be arbitrary anyway.