ModelButton: Add 'use-markup' property

Binds this property to the button's label, allowing a model button to
have text with markup.

This will be convenient for buttons like 'Online Accounts <sup></sup>'.
1 job for <span class="ref-name">wip/aplazas/modelbutton-markup</span> in 7 minutes and 48 seconds (queued for 2 seconds)
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passed #55218
build

00:07:48