wxAUI: Set minimum size on wxFloatingPane when floated

[Alex Bligh]

When undocking a docked item so it floats, the minimum size of the
wxFloatingPane is not currently set. The simple patch below fixes
this.

I'd just commit this as obvious, except we had to remove a line
which set the minimum size to (1,1) in LayoutAddPane which
others corrupts it. I'm unsure of the purpose (if any) of
this call, but perhaps there is something I've missed.

We've been using this in XaraLX for 3 months with no problems.

Please note the patch is rather more complicated if you
want to use it on release 2.6.3 due to a bug in gtk/window.cpp
which prevented emission of size events if the window did
not have scrollbars. This is fixed in CVSHEAD for both 2.7
and 2.6, but don't try it on release 2.6 as it won't work
properly (get minsize.p from http://www.alex.org.uk/wxAUI/index.html
instead).

Does anyone have any objections to me committing this patch?
Ben?

Alex