RGB Image: sample workspace
Bugfix: set RGB matrix editor values before connecting signals

The goal is to load all values before the handling starts.
Problem: set both offset X and Y to a nonzero value, select another function, select back the RGB matrix,
now Y is 0. If only Y is set, it it retained.
Cause: after setting X, the signal is emitted, setting Y from spin to image.
Bugfix: RGB Image editor used wrong signal for filename line edit.

This caused crash during manual edits of the filename