Use wxNotebook background colour for the tab row background in wxMSW.

Use wxNotebook background colour for the tab row background in wxMSW.

The free space near the tabs was always filled with the default light grey
colour, use wxNotebook background colour itself to fill it now.

Do it by changing the code in wxNotebook::OnPaint() to use the background
colour brush and ExtFloodFill() to paint over the default grey, instead of the
patterned background brush which is supposed to be used for the pages area
only and not for the tabs and which was not even taken into account anyhow as
the default window proc erases the entire contents of the DC we pass to it
anyhow (at least in the default top-aligned tabs case).

Also inherit the background colour from parent by default now, this ensures
that wxNotebook blends with its parent when its background colour has been
explicitly set by default, without the need to call its SetBackgroundColour()
explicitly.

See #13745.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74505 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775