Enable configuring allowed child content types

[alansemenov]

Part and Layout descriptors are currently not filtered in any way. When users select a descriptor for a new part, they always get the entire list of parts available in the application, even if they always use only a handful of parts in this specific content type.

To simplify this we will support filtering of part and layout descriptors by content type. Allowed content types can be specified inside a part or layout descriptor schema via regular expressions. Descriptors with a content type constraint should be matched against content type specified in API request sent from the client and filtered out if they are not allowed for the specified content type.

Sample descriptor:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<part xmlns="urn:enonic:xp:model:1.0">
  <display-name>My cool layout</display-name>
  <config>
    <allow-on-content-type>landing-page</allow-on-content-type> // only for content type `landing-page` and...
    <allow-on-content-type>${app}:*</allow-on-content-type> // ...for content types in this application
  </config>  
  <form>
    ...
  </form>
</part>