Revamped the events slightly

Payload data can now be properly injected into event handlers such as command triggers. Added a couple of new events to use for update triggers to the frontend instead of custom code, further decoupling the application. Movie rendering now also causes a frontend notification.
More documentation
Merge branch 'revampedEvents' into devel
WARNING: A lot of changes to the existing API and the event system.

This WILL break existing API clients and probably some event handlers too. I'm sorry for the disruptive changes, but I needed to rectify some decisions before they went too far utilized elsewhere to still be corrected.

 Basically this change completely removes the old API and switches it (same endpoint) with the new one, that's basically the existing AJAX API that the client uses, but way more RESTful and based on JSON (exception being the file upload).

 The event system has been revamped to carry more payload data (and in an extensible form as dictionary, to allow for later addition of attributes to single events), with the existing event listeners adjusted to also allow users to make use of this data in their consumers.

 Documentation has been greatly enhanced for the REST API (and is still being added to), the events will be documented here as well.