Allow user to visualise changes in convolver parameters

Convolver parameters in SubframeFindPeaks are now modifiable and changes are transmitted to the DetectorScene. The user can now visualise changes in these parameters and the threshold.

Removed some reproduced code for generating the filtered image from SubframeFindPeaks; this is now handled by DataSetGraphics which correctly handles all elements of the image, so now peaks, masks etc. can be superimposed onto the base image.

Resolves #572 (closed) and #590 (closed)