Add types to remixer core.

[miguelandres]

This adds a DataType class to remixer_core that lets you model types that can be manipulated by RemixerItems.

Types must be registered with the Remixer Instance and their default UI widget as well. These registrations allow us to refactor the logic to be more robust.

In a future PR these data types will be merged with remixer_storage's SupportedDataType, thus providing an easier path for extension.

Ultimately types make Remixer more robust altogether:

• Not depending on an implicit contract between UI and core to determine whether an ItemListVariable<Integer> is a color or a Number.
• Keeping the default UI logic in remixer_core, and minimizing the number of implicit contracts, simplifying the logic.
• Not depending on calling getClass on the current value of a Variable to find the class it should hold (this was very fragile in case there were subclasses) ...

[codecov-io]

Current coverage is 66.38% (diff: 52.04%)

Merging #83 into develop will increase coverage by 0.23%

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