[Mono-winforms-list] checked in fix for 80020

[Chris Toshok]

So I finally got the fixes for 80020 working both with jpobst's
ControlHandleTest.cs and all the current 1.1 tests (on my box, anyway.)
There are 4 failures in the 2.0 tests, but I haven't checked to make
sure those weren't happening before the change.

Part of the change included making set_Visible synchronous on X11.  This
has a *huge* impact on performance, obviously, most visibly in the
XplatUIX11.UnmapWindow case.  If that method is left async, thing are
more or less the same performance-wise.  With it synchronous, things
seem to slow down a lot.

This means we can go back through the tests and remove the NotWorking
category for tests which assume Control.Show() is synchronous, and we
should be able to start working on the other bugs which rely on this
behavior being fixed.

It's a large change, requiring lots of little things to be moved around,
so lots of things could obviously go wrong.  Hopefully things won't go
wrong though :)

Chris