[FEATURE] Add preview modes for composer - needs feedback!

This pull request adds a new QGraphicsEffect to QGIS for simulating grayscale, monochrome and color blindness (see http://hub.qgis.org/issues/9886 for the original inspiration). I've added it to the composer, so that interactive previews in any of these modes are possible. It should also be possible to add this effect to the map canvas so that map layer symbology can be tweaked with immediate feedback.

HOWEVER - there's an issue somewhere in the code which I can't track down (or replicate). In Nathan's testing switching on any of the preview modes stops the composer view widget from repainting (effectively freezing the control) until the effect is disabled. I've tested on a range of windows and linux environments but can't reproduce this or figure out what is happening. So, any advice on what is going wrong here would be appreciated