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>'.
4 jobs for wip/aplazas/modelbutton-markup-master in 21 minutes and 40 seconds (queued for 51 seconds)
Status Job ID Name Coverage
  Build
passed #54397
fedora-x86_64

00:06:34

passed #54398
win32
msys2-mingw32

00:03:52

 
  Flatpak
passed #54399
flatpak:demo

00:05:42

passed #54400
flatpak:widget-factory

00:05:30