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] Written JS unit tests,
* [x] Improved the visuals.

**THINGS TO BE DONE**
* [ ] Create content view of PlatformUI

Use the *return asap* form:

```php
if ( !$request->isXmlHttpRequest() )
{
    return $jsonResponse;
}

// ...
```

It has the advantage to lower the indentation, and hence let the code more readable.