This removes the remaining cases in 6f6070b5 by replacing them with a
simple placeholder selector.
`%button.flat.suggested-action` has been replaced by
`%selection_mode_button_flat`, because it's difficult to replace the
compound selector with a simple placeholder selector, and it doesn't
seem like a proper selector for `.selection-mode button.titlebutton` to
inherit.
The CSSs were generated with libsass 3.5.5 to minimize the git diff, but
I've confirmed that no warning happned with libsass 3.6.3 anymore.
See https://gitlab.gnome.org/GNOME/gtk/issues/2237
f7d57cc1eb