Bug 1111575 - Conditionally set mainTitle on mozL10N.ready. r=fcampo

The patch ensures we only add a data-l10n-id on #main-title if one has not yet been added. We use navigator.mozL10n.ready as the entry point to start the FTU, but when you select a different language mozL10n.ready fires again. Previously we had a race condition that allowed the #main-title to get set to Language even if the user was no longer on that screen.