Adopt S7 in most places

[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.

[teunbrand]

When we have an S3 generic we need for both S3 and S7 objects, we need to implement methods for the S7 objects in the S3 fashion, because using S7::method() will invalidate the S3 methods.

[teunbrand]

Older versions of R seem to get stuck on dispatch with +. We can get around this by implementing this + as S7 methods only (i.e. converting the +.gg method), but it makes error messages less informative. For example: Cannot use + with a single argument. i Did you accidentally put + on a new line? will become invalid argument to unary operator and also Cannot add <ggproto> objects together. i Did you forget to add this object to a <ggplot> object? becomes non-numeric argument to binary operator.

[teunbrand]

Add note to remove at some point

[teunbrand]

TODO: Remove class gymnastics
Clearly mark workarounds for R < 4.3.0