themes: remove various @extend with compound selectors

themes: remove various @extend with compound selectors

They are no longer supported by sass and broken with libsass 3.6.3
(https://github.com/sass/libsass/issues/3033)

This removes them by replacing them with a placeholder selector. This at
least brings the resulting CSS size down a bit so gtk can be build
again.

`%button.flat.suggested-action` has been replaced by
`%selection_mode_button_flat`, which is a more appropriate selector for
`.selection-mode button.titlebutton`.

The CSS was generated with libsass 3.5.5.

Co-authored-by: Christoph Reiter <reiter.christoph@gmail.com>
See https://gitlab.gnome.org/GNOME/gtk/issues/2237