Use the 128x128 icon for Desktop notifications

due for example to it being called by an event handler.

to ensure naming consistency, we have the same method on `ChatRoomView`.

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`.