compose: don’t isolate plugins.

Plugin isolation was already broken, anyways; EventEmitter methods
returned the context, so isolating them would have required wrapping
them; that seemed silly.
selection: don’t use the 'ready' event

The Selection module was the only one subscribing to this event, so
the event has been removed completely.
dispatcher: listen to composition events.
test: fix spacebar tests.
richMode/paragraphs: convert paragraphs after events

Previously, when inserting/updating paragraphs and sections, the
paragraph was converted to an element before the paragraphUpdate
event was emitted; as a result, any changes made to the paragraph
by event listeners were not reflected in the DOM. This fixes that
issue.
richMode: implement rich text editing.