create a new BeanManager#isInUse()

Sometimes it's useful to know if OWB really got used from an application. Depending on this we could implement performance tweaks, like e.g. disabling the WebBeansELResolver for non-CDI apps.

This would be very usefull in EE scenarios if the OWB jars are available in the classpath but CDI doesn't get used by a WebApp.

I'll just separate BeanManagerImpl#addBean() into a 'public' version for standard beans and a #addInternalBean for adding BeanManagerBean and stuff.
#addBean() will set a isInUse = true; whereas addInternalBean will not touch this flag.