Standardize the interface and behavior of command-related `Subject` instances

[telamonian]

Currently, all of the commands (open, cut, paste, etc) are associated with an rxjs Subject instance. The exact implementation is (as of v0.0.12) a bit all over the place. Let's standardize the implementation. One possible plan:

• All command-related Subjects live in ClipboardModel (might need a rename)
• Both internal tree-finder code and 3rd party integration code should follow the same patterns for interacting with the commands
  • To trigger the command, supply the value on which the command should act via next
    • eg fooSub.next(somerow)
  • To participate in the implementation of the command, subscribe via a callback
    • eg fooSub.subscribe(row => {/*whatever*/})

Pros:

• very flexible

Cons:

• 3rd party code that subscribes to a Subject will also be responsible for proper cleanup:

// subscribe
const mysubscription = fooSub.subscribe(somecallback);

// later, clean up
mysubscription.unsubscribe()