Changes behavior of the box selector ComboBoxes

This does two things:

1. It keeps the selection of `CB_BoxEnd` if `CB_BoxStart` is changed.
2. It doesn't reset the selection upon loading of a new save.

I really have no idea whether the reset of the range is wanted behavior or not, so feel free to just close. This is just my personal preference, I don't think it is objectively better.  
Althourgh 1 really makes sense IMHO, so if you don't like 2, I can create a new PR with just 1.