GTK WIn32 theme - make buttons with .flat class appear flat and buttons in toolbar non-flat if they don't have said class

Currently in the win32 theme, a button with .flat class has a non-flat appearance, and buttons in a toolbar appear flat irrespective of their class. This MR makes them behave similar to the buttons in Adwaita (those with .flat appear flat and non-flat buttons can be present in a toolbar)

before	after	Adwaita

fixes downstream issue https://gitlab.com/inkscape/inkscape/-/issues/2896