[WIP] EZP-23012: As a developer, I want to be able to extend the application view services

JIRA: https://jira.ez.no/browse/EZP-23012

# Description

The idea is to make possible to extend view services (and later others types of components) with the [YUI Plugin](http://yuilibrary.com/yui/docs/plugin/) system. Basically, the developer will be able to register plugins for some components in a *global* registry and those plugins will be dynamically added. Together with the configuration system proposed in #95, this will allow to cleanly extend almost anything in the Platform UI App.

# TODO

* [x] implement a plugin registry
* [ ] define a base class for the view service plugins
* [ ] reorganize existing view service code in plugins (for example object relation related code, content tree, ...)
* [ ] if time, add the same concepts to others types of components (apps, views, ...)

# Tests

unit tests + manual tests