Some simple interface changes

[davvidde]

I don't know if comments constraint is a widely spread practice but when I add or modify a constraint I often use these, so add a "comments" button in these dialogs it would be greatly appreciated. Also ad enable/disable constraint button would be useful.
I often use comments to group same constraints but alphabetic sort and group by constraint-type,  comments and insert order I think would be useful; could you add a preference dialog for these and others future settings?

Davide.

[Liviu Lalescu]

I know about these, should be done, but it is a lot of tedious work. Also, in each constraint's dialog, there might not be enough space for new buttons.

I added your words in the TODO.