combobox: Add a special class for the button in the combobox

combobox: Add a special class for the button in the combobox

That way, the GTK engine doesn't think that the general .button CSS
might potentially apply to it.
And because combobox button is overly complex and stupid, it cannot be
cached.

So buttons thought they cannot ever cache anything because they might
suddenly end up inside a combobox without noticing and then they'd need
to round their corners differently. Of course they're just regular
"Remove" buttons like all the other 100s of "Remove" buttons in
gnome-software. But hey, better not cache anything for them and
recompute their CSS every time the :hover state changes on one of the
rows.