Internal refactor of figures and views

[nvaytet]

Internally, it has gotten quite confusing what is a Figure and what is a View.
Figure currently refers to multiple things:

• the object returned by the plot function (and similar) which contains another "figure" inside, and maybe a toolbar also
• the "figure" mentioned above which could be a FigLine or a FigImage, which contains a canvas
• the underlying matplotlib figure, contained in the FigLine's canvas.

This is getting difficult to navigate.

This PR proposes to move things around and rename for (hopefully) improved clarity:

• the FigLine and FigImage and now called views: LineView & ImageView
• the object returned by the plot function is still a figure. It inherits methods from the common Figure mixin, which in turn inherits from the BaseFig (now just a mixin class)
• the underlying matplotlib figure is still called fig

[YooSunYoung]

In case we are not keeping the callable anymore,
should we make sure if the _view.__class__ is the View in def __init_figure__(self, View, *args, **kwargs):...?

For example,

class FigBase:
  def __init_figure__(self, View, *args, **kwargs):
    self._view = View(*args, **kwargs)
    if not self._view.__class__ is View:
        raise TypeError

Cause, you might have something like

@dataclasses
class Figure:
  width: int
  height: int

def create_figure(size) -> Figure:
  return Figure(size[0], size[1])

fig = InteractiveFigure(create_figure, size=(10, 10))

In this case above, it will create the fig as intended, but copy_figure will not work.

Or is it too much of worry...?

[nvaytet]

I don't think any user should every be constructing InteractiveFig manually, so I would say in this case it is not worth worrying about?