bug#38296: Allow Option key to be modifier for non-char key and mouse events

[Mattias Engdegård]

It was pointed out to me that it would be useful to set this behaviour 
individually for either Option key.
This can be accomplished in various ways. For example, 
ns-alternate-modifier-char-entry could be changed from a boolean into taking 
either of the values nil, left, right or both, indicating to which Option key 
it applies.
Or we could simply use two separate boolean variables. I'd appreciate advice 
from the NS maintainers about what practice is best.