usability improvements

[steveburkett]

I'd like to see:

(1) fix undo:  with my sequence diagrams, undo some times works, sometimes doesnt (or it has an indeterminate effect)
(2) if i have a [alt] frame within another frame (like loop), i have to have the z-order right to select anything. this is cumbersome.  also, drawing messages to the underlying lifelines is complicated by the z-order.  
Note that Visio handles this difficulty much better.

Thanks  

[sargasso]

Ditto with nested regions.  It is impossible to use the context menu "Selectable" with three levels of regions.  
bruce