Simplify and correct MSW selection of background brush for children painting.

Simplify and correct MSW selection of background brush for children painting.

Check for the transparency of the window background in MSWGetBgBrush() itself
and let MSWGetBgBrushForChild() just return the brush to be used.

This required adding a MSW-specific hack to wxPanel to allow inheriting
background of wxNotebook by its children which is slightly ugly but is exactly
what we need to do in this case and makes the rest of the code much simpler.

Also add a possibility to set the background of the page containing the
control being tested to the widgets sample to test for background inheritance.
This shows problems with wxSlider and wxRadioBox backgrounds but they're not
new with this patch.

Closes #12057.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64372 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775