Fixes #29, #30.

* We used to employ a pretty shady method of ensuring the gMsgCompose object (and the editor) is ready for us to use. No longer! We now register a listener which is notified when the message body is ready - at which point gMsgCompose is usable. This listener is registered in the handler for the "compose-window-init" event. It's magic!
* Specifically, we no longer need a time-out from whatever event listener we run; and we no longer register event listeners for "load" and "compose-window-reopen".
* When calling the `directionCheck()` function on reply message bodies, we were - somehow - not passing the NodeFilter object. Crazy! That bug must have been manifesting for years.
* Trying to be nicer about adding a stylesheet (the quotebar fix) to the editor
* Dropped some commented-out code relevant to earlier TB and SM versions
* Some `let`'s-instead-of-`var`'s
* Avoiding some code duplication in `setInitialDirection()`
* The code we used to obtain the currently-loaded message's URI for a debugging message is no longer working... for now, just skipping the printing of the URI, in case it's unavailable.
* Remove some unnecessary newlines in `console.log()` calls
* Fixed out-of-date variable name in a `console.log()` debug message in `bidimailui-display-logic.js`
* Renamed some event-listener and related functions.