qmlstatemachine: Rename StateBase to State

As discussed with Brett Stottlmyer and Alan Alpert.

Add a section about the implications when importing both
QtQml.StateMachine
and QtQuick in one single QML file.

Change-Id: I8755f4b578e2a6ff4c2377c7a8a0b996ba9b7129
Reviewed-by: BogDan Vatra <bogdan@kde.org>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>