Feature enhancement: eyedropper for GHT tool

Implements an eyedropper tool for the GHT tool that can be used to set the value of SP from the current selection. This greatly eases setting the stretch focal point precisely to a particular intensity value corresponding to (e.g.) the diffuse arm of a galaxy. SP is set to the mean value of the selection, weighting each channel evenly. Minor tweak to image_interactions.c to allow for making a 1x1 selection by a simple click without drag on the viewing area, this is to allow for precise selection of a pixel to set as SP.