Blender Git Loki

Sequencer: remove strip's color balance in favor of modifiers

Having two ways to control color balance now seems a bit overkill
and not clear.

Removed old Color Balance settings from the interface and logic,
added versioning code to convert this settings to modifier.

Unfortunately, since color balance was a pointer, it's not actually
possible to preserve compatibility of old files saved in new
blender and opened back in old blender.

Hopefully there's no regressions :)