EZS-14 - Build "Create Content" module in the sidebar of PlatformUI

Created a sidebar module to allow a user to create a new content.

https://jira.ez.no/browse/EZS-14

**THINGS DONE**
* [x] Fetching content types from the system,
* [x] Filtering a list of content types with search query and selecting content type group.
* [x] Completed the code documentation,
* [x] Prepared JS unit tests,
* [x] Improved the visuals,
* [x] Create content view of PlatformUI

you won't set any route in the plugin, the content (and its first version) is created when the user chooses a content type (you don't display anything here, so you don't need a route) and then he is redirected to edit this content.

Let me detail a bit how this should work:

* the user chooses a content type in the list
* this is translated into an application level event, the event facade contains the identifier of the content type (and maybe a hardcoded language)
* this event will bubble to the location view service
* the location view service has a ContentCreate plugin
* this plugin subscribes to the application level event mentioned above
* when the event occurs, the plugin creates the new content with the info in event facade (content type identifier + lang) and by using what the view service (parent location)
* the new content is created then the user is redirected to the `editContent` route for the new content

In a second step, we'll have to change a bit the content edit view service, because it always creates a new version while in the case of the content creation, the version to edit is already there, but first implement the step above and we'll see how we manage the version after (it's not completely defined yet).

For the plugin, you can take example on the Y.eZ.Plugin.ContentTree plugin on how to register a given plugin for a given service and how to listen for an event that bubbles to the service.