`Gridplot` -> `Figure`

[kushalkolar]

closes #457

This PR breaks everything :smile:

• [x] Gridplot renamed to Figure
• [x] Plot is removed in favor of just doing Figure() since the default shape=(1, 1), i.e. "gridplot" with a single subplot
• [x] Frame merged into Figure
• [x] RecordMixin -> FigureRecorder
  • record is now an instance of Figure, use composition instead of inheritance, I haven't tested that this works yet, right now it's still a very experimental feature
• [x] _frame module renamed to output, some reorganization of what is public/private within that module to make it more logical
• [x] updated all the examples and tests
• [x] update ImageWidget

Still need to do this for ImageWidget, waiting for #459 , rest of it is good to go and tests are passing on my end :partying_face:

[kushalkolar]

when reviewing don't be scared by the size of the diff, it's mostly stuff being moved around, and the changes to the examples are mostly renaming stuff from plot -> figure, gridplot -> figure :smile:

[kushalkolar]

Idea: if figure.shape == (1, 1), should be make figure.__getitem__ just pass to subplot.__getitem__ so the user doesn't have to do figure[0, 0] :thinking: