This commit fixes a regression from #18596 where the "Add image" button
styles broke because it's a labeled toolbar button but the labeled
toolbar button CSS rules were only scoped to the secondary toolbar.
However, nowadays labeled toolbar buttons are also used in e.g. the
editor parameters toolbar, and this highlighted that we didn't clearly
encode the concept of labeled toolbar buttons in the CSS so far.
This patch extracts the CSS rules for labeled toolbar buttons that were
previously limited to the secondary toolbar into a dedicated generic
class that can be applied on top of any unlabeled toolbar button to
convert it to a labeled toolbar button, regardless of its position in
the DOM. This also makes the distinction clearer in the HTML, and the
new CSS scope for the toolbar buttons lays the foundation for combining
the other toolbar buttons rules in there later on.
Co-authored-by: Calixte Denizet <calixte.denizet@gmail.com>
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