Welcome: Move IWelcomePage to core plugin

IWelcomePage now uses Core::Id, and it can only used by plugins anyhow.

Change-Id: I21f035fee9dec66830d3aa7b7eb2242f703095cb
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>