Increased opacity of ROIs

Increased the opacity of ROIs so that color differences are more readily
apparent.

Also changed mechanism of beenViewed image to simply update the cdata
instead of deleting and creating new image.
Added white and black sliders to adjust contrast

Added sliders to adjust image contrast. Will only work for horizontal
screens.
Added shift-tab hotkey

Added shift-tab hotkey to cycle backwards through rois and fixed bug
with focus.
Created secondary figure for traces

Creates a secondary figure for the extra figures (slope, traces,
neuropil subtraction). If figure is closed, new individual figures (old
behavior) will be created.