Formalise 'plugins' - start with Kendo

[cursork]

In 6c37c8c we removed Kendo and got 54 less dependencies and a 30% smaller file size in the JS build. That being said, we want to be able to make EWC extensible for user-provided components.

My suggestion:

• Components are external repositories containing APL definitions and JS to be injected
• The user will simply call a function on initialising a Form to include some JS (or it could be included in the WC for the Form)
• That JS needs to simply call something like window.ewcRegisterComponent(...) which means the ewc-client will know what to do with WC requests
  • We'll also need hooks for customised WG, WS, NQ