Add automatic metatype registration for invokable methods.

Add automatic metatype registration for invokable methods.

This works similarly to the automatic registration for Q_PROPERTY types,
but in this case it mostly affects the need for users to
call qRegisterMetaType<T>() before using queued connections
with methods using non-built-in metatypes, or before using invokeMethod
manually.

Change-Id: Ib17d0606b77b0130624b6a88b57c36d26e97d12d
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>