Expose CyLeaflet subcomponent IDs + Add additional documentation of CyLeaflet design

[emilykl]

Description of changes

• Expose the subcomponent IDs as properties of the CyLeaflet object. This makes it easier to use these subcomponents as callback inputs and outputs. Among other things, this makes the existing solutions described in [BUG] CyLeaflet: Updating Leaflet tile layers does not change max zoom of CyLeaflet #205 and CyLeaflet: Provide way of updating Dash Cytoscape elements via callback #208 easier to use.
• Update the CyLeaflet demo app to demonstrate usage of these subcomponent IDs.
• Add an additional document explaining the design of the CyLeaflet component, to make future development easier. See the rendered version here.

Pre-Merge checklist

• The project was correctly built with npm run build:all.
• If there was any conflict, it was solved correctly.
• All changes were documented in CHANGELOG.md.
• All tests on CircleCI have passed.
• All Percy visual changes have been approved.
• Two people have 💃'd the pull request. You can be one of these people if you are a Dash Cytoscape core contributor.

[alexcjohnson]

Just to make it easier than cyleaflet_instance.ids["leaf"]? I guess this way you get autocompletion?

Regardless, in order to use this you needed to pull cyleaflet_instance out of the layout... which might not always be possible, I wonder if it would also be worth making a cyto.CyLeaflet.get_leaflet_id(cyleaflet_id) etc that would construct the PMC ID for you - and then you could even call cyto.CyLeaflet.get_leaflet_id(ALL) or (MATCH) if you wanted to construct your own PMC?

[emilykl]

Yeah exactly, this way you get autocomplete and it looks cleaner than accessing a dictionary.

I see your point -- I'm open to switching this to exposing static functions instead.

[alexcjohnson]

I don't think it needs to be one or the other - this pattern is very nice when it works, but we can add the functions as well to cover the cases when it doesn't work.

[Farkites]

lgtm!