Using overlay, it is impossible to override any of the translated texts other than the title and description (which still defaults to 'wildfire' :S).
Overlay functionality should allow for all texts from i18n files to be overridden.
i18n files can now be modified by files in custom/i18n/. Files are merged, where the overlay values have preference over the original values.
This commit removes support for the (poorly implemented) overlay.properties file.
Implemented in: https://github.com/igniterealtime/Openfire/pull/294