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.
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.
Don't call trimChats in onChatBoxAdded event. Reduntant because it will be
called after maximization and after being shown.
Add new method getShownChats and don't trim if only one or zero chats are being
shown.
Don't trim chats when in responsive mode.
Don't call trimChats unnecessarily in the render method of the trimmed chats
thingy.
Previously views were only hidden, but not removed. This was an unnecessary
"optimization" which introduced unnecessary complexity.
Problem solved was that closing minimized chats didn't dereference the model
for hidden "normal" chat views, causing an exception to be raised when trying
to log out.
