Bug 1042319 - [Gallery] Update gallery edit UI r=djf

I updated the comments in the code to add more details on why we need forceSetCropRegion .  The purpose of forceSetCropeRegion is to update source rectangle crop region with the crop settings in editSettings object. We need this when user cancels after clicking back to original button in crop edit tool. Since back to original resets source rectangle to original image and discard crop preview, we need to revert back to previous cropped image by explicitly calling resetpreview and forceSetCropRegion

resetCropRegion resets source rectangle  to original image size and not to previous cropped size in edit settings.