Add fontsize aesthetic to geom_text

[thomasp85]

A common source of confusion is the scaling of text in layers and how to match sizing between that and theme element text. This is mainly due to size being interpreted as cm to make it in line with the sizing of other geoms.

Would it make sense to introduce an alternative aesthetic to geom_text that was interpreted as 'pts' instead which, if given, would take precedence?

Maybe it shouldn't even be an aesthetic, but a switch, I dunno... But this comes up often enough that we should discuss how to address it...

[teunbrand]

I think having a switch is more lightweight than a separate aesthetic. Should the only option be centimeters/points or should also other grid units be supported? size_metric = "cm"/"pts" might be a good name for the switch.