Window manager: set WS_CLIPCHILDREN state for docked toolboxes

I had endless grief when trying to do this before, and I swore I'd never
revisit since the matrix of OS versions and theme settings is too
painful to handle... but like most things, I couldn't help myself from
giving it another try.

The benefit of child clipping is a huge reduction in flickering when
moving and resizing toolboxes under Aero.  With my present work on
making the Layer toolbox resizable, it was as good a time as any to try
and fix this behavior.

I *think* I've solved the previous problems, and testing under a variety
of themes on both Win 7 and XP shows no obvious trouble.  It's possible
that child clipping could also be used with floating windows, but as
that use-case is rare, I'm not going to attempt it just yet.
Layer toolbox: make it user-resizable

This was every bit as unpleasant as I'd anticipated, but I think
everything's back in working order, save for two known issues:

1) Floating mode layer box isn't resizable.  Some extra plumbing is
needed to make this workable.
2) The resize cursor doesn't always appear when it should.  This is due
to some messy interactions between two simultaneous pdInput instances;
one for the form, and one for the layer toolbox.  I'll revisit this once
I've done some additional homework on cursor handling options.