Provide a label attribute for toolbar buttons

henryrequested to merge
henry/tor-browser:add-label-to-toolbar-buttons-bug-41283 into tor-browser-102.2.0esr-12.0-3

Also tidy up the code to ensure the buttons are fully initialized whether they are part of the DOM on load, or if they are added later through customization.

Closes #41283 (closed)

Closes #41287 (closed)

Edited by henry

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