Weighted tracks

Added a weight slider to track which defines
how much particular track affects in a final
reconstruction. This weight is for sure
animateable.

Currently it affects on BA step only which in
most cases will work just fine.

The usecase of this slider is to have it set
to 1.0 most of the time where the track is
good, but blend it's weight down to 0 when
tracker looses the track. This will prevent
camera from jump.

Tutorial is to be done by Sebastian.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60948 954f8c5b-7b00-dc11-b283-0030488c597c