Push all pre-created sub pages of a KCM
Currently if a KCM calls kcm.push() inside the Component.onCompleted it gets lost.
This will happen when the mainUi is created, which is before we've added the mainUi and connected the signals.
Even if we re-arranged this method, KCModuleLoader has already queried mainUi (a non-const getter that constructs the main UI) before we get into the constructor of KCModule and connected to pagePushed.
The intended use-case is the UsersKCM which wants to start on the second page of the KCM.