Add `italic`, `fontweight`, and `fontwidth` as aesthetics for `geom_text()` and as arguments to `element_text()`

[thomasp85]

As we all know the fontface spec is horribly limited. I think we can figure out a way to leverage systemfonts to circumvent this, though device support will be limited (quartz, ragg, and svglite AFAIK)

However, it wouldn't impair on current behaviour so that really shouldn't be a blocker

[teunbrand]

As we're converting to S7, do we want to already add these as properties to minimise backward compatibility pain in the next release? Alternatively, should we just allow a grid::glyphFont()?

[thomasp85]

What kind of compatibility issues do you envision by adding properties at a later point?

I don't think we should go the glyphFont route -- that would prevent us from eg scaling weight

[teunbrand]

Say some extensions {foo} defines an element_text subclass. Their class definition is fixed upon build time. ggplot2 updates their element_text definition with new properties. The {foo} subclass is now incomplete. If we include these properties now, the {foo} subclass will not be incomplete later.

I don't think we should go the glyphFont route -- that would prevent us from eg scaling weight

Fair point

[thomasp85]

Good point. So maybe add them to element_text() but don't have them in the constructor etc