Don't record (de)activating change tracking on undo stack

[Daniel]

LyX has the peculiarity of treating the (de)activating of change 
tracking as something that is recorded on the undo stack.

One of the problems I ran into with this is that it unexpectedly killed 
the redo function when I activated change tracking, i.e. I undid some 
changes and activated the change tracking in between and this changed 
what could be redone.

The only other app I know of that does this is Apple Pages. But I am not 
sure whether Apple's change tracking should be taken as a thought 
through feature, e.g. it is impossible to de-activate change tracking 
without accepting all changes. Go figure!

Daniel