redesign of GUI event handling

[John W. Eaton]

I checked in a series of changes to simplify the GUI event handling
code.  Now each event is processed in a single function instead of
using a two-step approach.  All the dynamic_casts that were used to
decode the type of event in the event_handler callbacks have been
eliminated.  Instead, we now use an event_queue object similar to
Octave's  unwind_protect stack (they are both now derived from a
common base class) and class methods and (optionally) some associated
data may be registered on the event_queue to handle the callbacks.

I don't think anything was lost in this conversion, but I may have
missed something.  Please report any problems you notice.

jwe