Fix inheriting parent background colour in wxPanel in wxMSW.

Fix inheriting parent background colour in wxPanel in wxMSW.

wxPanel didn't inherit the background colour set for its parent any more in
wxMSW because its HasTransparentBackground() didn't return true unless the
panel was a child of wxNotebook. This was wrong because not only themed
notebook background should be inherited but also any solid background
explicitly set for a panel parent.

Fix this by returning true from MSWHasInheritableBackground(), which is used
by wxPanel::HasTransparentBackground() under MSW, if the window has an
explicitly set and inheritable background colour.

Closes #13487.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69348 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775