Add flag to enable the inclusion of full url in the tab hover preview

[yh0xe]

Use case:

When tabs are unloaded, and a site have indistinguishable titles between multiple pages, it would be nice to simply hover over the tab and see the full URL.

Relevant code. The label should break into 2 or 3 lines, before overflowing, similar to title. Ideally, the implementation should allow further tweaking in a "userChrome.css".

[Jon]

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