Create a Back Door to mx::core::DocumentPtr

[webern]

A user may want to use mx::api for most things, but need to get at mx::core::DocumentPtr for some tweaking. Provide a way to do this without exposing mx::core symbols in any mx::api headers.

Requested here #44 but leaving that issue to be about the specific key signature issue.

[webern]

Looks like I already did this:

mx/Sourcecode/include/mx/api/DocumentManager.h

Line 78 in 38f38fc

mx::core::DocumentPtr getDocument( int documentId ) const;

The trick is that DocumentPtr is only forward declared in this file. So as long as you don't touch that function, you don't need any headers from private/mx/core. If you include those private headers, then you can use this function to access the underlying document (and change it since everything is a sharedptr).