[D&D] Make new "meta" state slice to keep track of things we don't want to save and load

[joshuarrrr]

After #1870, we need to further expand and refine the behavior of hasUnappliedChanges.

We should keep track of whether the canvas is in a savable state or not, and only enable the save button when the current state is valid and the canvas is rendering the current state. The best way of doing so may be creating a new "meta" state slice, which we can use to keep track of this (and other properties) that we don't want to include in save and load payloads.

• Save button should be disabled with an empty canvas, or any time there's not yet enough required information to render a vis
• Save button should be disabled when agg editing suspends realtime updates
• Save button should be disabled when there have been no changes to a loaded vis
• Tooltip and/or toast should provide info on why the vis is not saveable and how to fix
• Savable state should not be part of Visualization or Style slices, because it should not be saved/loaded.

[joshuarrrr]

New metadata slice was created in #1953, #1965.