Refactor QWindowsXPStylePrivate::drawBackground() and helpers.

Pass the DC obtained drawBackground() to drawBackgroundDirectly(),
saving a call to hdcForWidgetBackingStore() to find it.
Pass the error code from the Win32 API DrawThemeBackground()
to the calling functions.

Change-Id: I1c25241f53d87e4429ad924f7b2e29da215d7e4f
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>