filechooserentry: Reorganize function

filechooserentry: Reorganize function

Setter functions the way I code it look like this (in order):
1) Figure out if the value changed. If not, exit early.
2) Clear the old value (ie unref stuff, disconnect signals
3) Copy the new value
4) Set up things about the new value

This reorganization does that.

And by doing that, it even reduces the amount of code and the amount of
branches (and with it, nesting) needed.