GridPlot -> Figure as the only user-facing layout  #457

Layouts currently has a lot of multiple inheritance, this is partly due to us having a separate Plot and GridPlot class. After meeting with Almar I propose:

1. New Figure class which merges GridPlot, Frame and RecordMixin.
2. Remove Plot in favor of creating a Figure with shape (1, 1), i.e. one subplot. We can have shape=(1, 1) as default for Figure

@almarklein Does this make it easier to have axes etc.? Anything else to add?