Move init of RosterGroups collection and presenceHandler registration out of
the controlbox and rosterview plugins and into core, since this code is
generally useful (indeed necessary) even without those plugins.

In some cases the element is not visible,but we want to set 'display: None'
regardless, for example in cases where you only want one single chat of
multiple to be visible at any one time, while hiding it behind a sidebar.

to indicate whether the room should be created if not found.

By listening for this event before registering event handlers, other plugins
can first get their overrides of those handlers applied.

because the method it was delegating to was applicable to private chats only.

* Empty chat area doesn't resize when hiding occupants.
* Properly change icon when toggling occupants

* online users count issue-656

* change log entry added for issue 656

when trimming chats.

by not attempting to load `optional_dependencies` via require.js.

Instead, we just expect them to be plugins and to have been loaded already.

and use that in the tests, instead of a callback.

This change refactors out the plugin code from converse-core into
src/converse-puggable.js

Additionally, plugins now have an optional attribute `optional_dependencies`
which is an array of dependencies which are "nice-to-have" but not essential.

Work has also been done to ensure that a plugins' dependencies are first loaded
before the plugin itself.

Will have to come up with some other way to disable it in cases where it's not
needed.

when strict_plugin_dependencies is set to `false`.

To make overridding it easier.

We already emit an event when the roster is fetched from the XMPP server,
similarly, it would be useful to know when the roster was instead fetched from
the cache.

if it's defined, then we want the extra check that the jids match, otherwise
we might be attaching to someone else's session.

Don't <show> 'offline'.
Only include status_message when explicitly passed in as a string.