Create meta-action creators (part 3 of the Redux refactor)

[jimporter]

The list/cache reducer and the editor reducer use some of the same actions, which is a violation of the ducks pattern, as well as being very bug-prone. To fix this, we should create actions that apply to just the list and just the editor, then have meta-action creators that dispatch the appropriate actions in response to a user interaction. For example, clicking on an item in the list should (prompt to) close the editor as well as updating the currently-selected item.