Reimplement classes in S7

[teunbrand]

This PR aims to fix a large part of #6352.

Briefly it implements the following S7 classes:

• class_ggplot
• class_ggplot_built
• class_mapping
• class_labels
• class_theme

I've put in the usual extractors and replacers ([, [[, $ and their <- methods) for the ggplot/ggplot_built classes for backwards compatibility. The code internally now uses @ though, so that we can deprecate the usual extractors/replacers later.

[thomasp85]

LGTM

[teunbrand]

I think it is necessary to make ggplot() a S3 generic again. I have the following reasons:

1. Extension have S3 methods for the S3 generic.
   • You cannot write S3 methods for an S7 generic.
   • There are too many extensions to force them all to use S7 right now.
2. Unlike S3 ggplot_build(), which we plan to replace with S7 build_ggplot(), the ggplot() function is much more user-facing. I don't think we want to phase-out ggplot() for other function names.
3. If people want to use S7 methods, they can register them for the S3 generic without any issues.