Ticket #223

Hi Guillaume,

first of all: you are quite hardworking today.

I thought about #223 because I really don't like app crashes.
WxWigets tells me that frames opened before app main loop are not destroyed.
(http://docs.wxwidgets.org/2.8/wx_wxappoverview.html#wxappshutdownoverview)

So I see there two possibilites:
_1 Say this is feature like wxWidgets does and care about not unchecked
accessing frmMain in frmQuery which is not null but invalid in frmQuery
_2 Close descendant frames on frmMain exit, by either delegate opening
to frmMain so this is done by default or take care about in frmMain::OnCLose
     or pgAdmin3::OnExit
What do you think about this.

Regards Steffen