Add optional i18n parameters to MWDialog titles

This change allows for the Username to be sent in MWBetaWelcomeDialog
title so the i18n message can support {{GENDER:}} parameters.

This change arose from the need to provide GENDER support when translating
the welcome message from Change-Id: I6a3da40b286bb1abd2aff360dab3b386deb13420

Change-Id: Iae70fbaa5be822d571a0d75704cfeca4eabd2fe8