Motion tracking dopesheet

Highlight background depending on number of tracks existing on frame.

This is not so much mathematically accurate displaying where things
shall be improved, but it's nice feedback about which frames better
be reviewed.

Bad frames are tracks < 8, highlighted with red.
OK-ish frame  are 8 <= tracks < 16, highlighted with yellow.

Could be some artifacts with color region start/end, this is a bit
unclear what exactly expected to be highlighted -- frames are
displayed as dots, but in fact they're quite noticeable segments.

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svn merge -r54572:54573 ^/branches/soc-2011-tomato



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54750 954f8c5b-7b00-dc11-b283-0030488c597c