This adds color management controls to the sequencer space, which are applying on the displaying image. Currently used the same UI approach as was used for other areas, but this would probably be cleaned up in the nearest future. This required a bit of hackish stuff to be done, Namely color management for the sequencer requires one extra step in the conversion chain to convert sRGB float buffer to linear space. This was made by setting special flag in ImBuf. Later this could be done using input color space for the ImBuf probably.