Track ggplot2 version in plot

[teunbrand]

This PR aims to fix #6714.

Whenever a plot is created, the new version property is populated with the current version.
If a plot is to be rendered, an error is thrown if the plot's version is not the same as the current version.
This should typically occur when a plot has been saved to disk with a prior version of ggplot2, and now no longer works.

There are a few open questions:

• Should we do this at all?
• Should we throw a warning instead of an error?

We can also do a revdepcheck to weed out all the packages that misbehave.