Implement correction.py fields completion

[bergentroll]

• When typing in filed1, field2, field3 combo lists, there is a prompt with matching variants. <Enter> or <Down> to open list and select.
• Fix clean3 button key.
• Add some errors handling to prevent crashes.

Also there is if ... elif ... elif ... instead if ... if ... if ... for event check in the mainloop because it is more efficient (in such a way comparing stopped after a first match).

[bdhrs]

excellent thanks. i learned a lot from reading through your improvements.