Fix minimizing by listening to change event
Fixes around showing/hiding and minimizing/maximizing
Don't trigger a `show` event on the model to show a chat, instead rely
on the `hidden` attribute which causes a rerender and will show/hide
chats.
The eventual goal is to avoid UI-related stanza processing if the relevant chats
aren't in the DOM.
With the current architecture, chatboxes are created (and the stanzas
related to them processed) even if `#conversejs` isn't in the DOM.
* Initial work on making controlbox an element
* Create a shared base class
* Ceate ChatBoxViews proxy
* Update sass now that certain classes are moved to converse-chats element
