Improving and sanitizing the reference selection code.
This fixes problems where formula references got incorrectly remembered as a selection upon editing a cell, changing a wrong cell and causing many problems.

Furthermore, enabled signal calling when doing reference selection list updates (no idea why Stefan decided to disable them).
This means that the colored boxes around referenced cells within a formula are actually displayed again.

Also unified the two editing widgets - each of them was using a different code to update cells and such, which led to a mess. Now one of the widgets simply relays all keyboard events to the other, who updates the text in both.
This greatly improves KSpread's behaviour when editing cells.

Some issues still remain - most notably, selecting the text in the toolbox editor will work oddly. I will fix this soon.

So few lines of code, so many improvements.
Yes, I'm good ;)