Changes to NSControl

[Fred Kiefer]

While working on the fix for the infamous selection error, that is 
caused by selecting the text of a non editable cell, I noticed a few 
more things on NSControl that would need changing. My fix for the 
original error is simply to make the text object editable while setting 
the text and switching back afterwards again. Perhaps the same is also 
needed while setting up the field editor, as some of this methods also 
check if the NSTextView is editable?

One of the strange things on NSControl I noticed is, that in the method 
mouseDown: the action mask of the cell is changed before calling the 
tracking code. Now this is the only place where the action mask is used 
at all, so resetting it looks pointless to me. I would like to just drop 
this code, that was probably a workaround for a problem now long gone.

The second oddity is the way NSControl implements the field editor 
methods. They are basically empty, although the documentation describes 
them in detail. Instead we have them implemented on the subclasses 
NSTextField, NSMatrix and NSTableView. I would suggest to move those 
implementations up to NSControl. The drawback of this is, that we also 
need the _text_object ivar in NSControl, which will never be used by 
some of the subclasses, but this is a minor price we have to pay.

If nobody opposes, I will do all of those changes.

Fred