serve: Rewrite of the Server-Sent Event code for build notifications. At the moment the server monitors the asset directories, and notifies the browser when an asset has changed and has been re-processed. * Fix issues around long-running requests/threads which mess up the ability to shutdown the server correctly with `CTRL-C` (see comments in code). * Move the notification queue to each SSE producer, to support having multiple pages open in a browser. * Add JS/CSS for showing quick notifications about re-processed assets. * Add support for hot-reloading CSS and pictures that have been re-processed.

PRIORITY_FIRST = -1
PRIORITY_NORMAL = 0
PRIORITY_LAST = 1


class Formatter(object):
    FORMAT_NAMES = None
    OUTPUT_FORMAT = None

    def __init__(self):
        self.priority = PRIORITY_NORMAL
        self.enabled = True

    def initialize(self, app):
        self.app = app

    def render(self, format_name, txt):
        raise NotImplementedError()