Clean up attribute inheritance for theming

[jkrumbiegel]

With the new figure and the larger role of Axis types our old model fails in unexpected ways, so we should think about how to clean it up.

The premise should be that (almost) everything is themeable, every plot type, every Layoutable like Axis, Colorbar, etc.

Some places which involve attributes that we can manipulate are:

• the const default minimal theme
• the current_theme via set_theme!
• the Figure has an as of now completely unused attributes field
• the Layoutables directly at creation
• Plot types directly at creation
• Scenes underlying Figure and Axis

Because layoutables used to be put directly into a scene, they want to inherit possible attributes from that scene. This is still the case even with Figure. Actually, the keywords passed at creation of the figure are currently just forwarded to fig.scene and the layoutables grab them from there.

Layoutables check for an entry with their own type name in the theme like Axis = (xticks = ... or Colorbar....

Plots had that behavior at some point but it doesn't seem to work anymore to theme scatter with a keyword in the theme. Something's broken there.

Basically, we need to come up with some graph that describes how attributes flow from the top to the bottom. Any ideas?

[ffreyer]

Can this be closed?