This converts the tabmodalprompt binding to a class, to be constructed along side with the element
by TabModalPromptBox.
TabModalPromptBox will keep the instances in a map and pass it to the callers, instead of the element.
The tests and callers can access the class instance by passing the element reference to the map.
Differential Revision: https://phabricator.services.mozilla.com/D15505
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rename : toolkit/components/prompts/content/tabprompts.xml => toolkit/components/prompts/content/tabprompts.jsm
extra : moz-landing-system : lando
bd38b39f8a