Rename internal model classes with 'Impl' suffix

[spoenemann]

This has been discussed before for quite some time, now I'd like to make a step forward in preparation of #231: The internal model classes like SModelElement, SNode etc. have the same names as the external model interfaces defined in sprotty-protocol. This is a source of confusion for users.

Historically, the external model interfaces had a Schema suffix to disambiguate, but I removed that during the move to the sprotty-protocol package. I now regard the external model as the primary interface. The internal model is used primarily at the diagram configuration with InversifyJS (configureModelElement) and as arguments to custom services such as views.

I propose to use the Impl suffix for internal model classes and deprecate the previous symbols. I would release v0.14.0 with this change (and maybe others) so users can migrate gracefully with the deprecation notices. Then I'd remove all deprecated code when we shift to v1.0.0.

[dhuebner]

I think the Impl suffix fits well and makes things clear.

Review all the rename, copyright and quotation mark changes is not really necessary I suppose.

[tortmayr]

LGTM! 👍🏼