st: Do not use markup in button labels

st: Do not use markup in button labels

It sounds strange that buttons would admit Pango markup
in their label text. It sounds better to default to plain
text, and let users wanting markup to access the child
directly.

This also makes the first label being set consistent
with later text being assigned wrt markup usage, since
the non-markup ClutterText API is used if the child is
already present and of the expected type.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3149>