pgAdmin beta fixes

Hi all,

I've found and fixed (or reported to wxWidgets) a number of issues in
pgAdmin beta 3 over the last few days. You've certainly been keeping me
busy with corner cases and hard to reproduce stuff! Anyway, here's a
quick rundown on some of what I've done.

- Fixed a bug in the query tool which could cause a crash or only
partial deletion of rows when deleting a large block in the Edit Grid.

- Fixed a bug in pgQueryThread. This could cause a crash when loading
data into the Edit Grid, or sometimes just left the grid empty with the
status bar saying 'aborting.'

This one was actually quite a nasty thread synchronisation issue that
could easily have caused a number of problems, including Erwin's export
bug, and George's long running query death.

- Reported the <return> acts like <tab> in dialogues issue to wxWidgets
(see
https://sourceforge.net/tracker/?func=detail&atid=109863&aid=1582051&gro
up_id=9863). The possibly-related mousewheel issue seems to have vanised
on my system at the moment.

Can I send you all an updated .exe for testing before I release beta
4/rc 1?

Thanks, Dave.