Improve registries of available resources

• provide registries for all resources manipulated by studio
• improve the resources registration using .json objects, clearly identified for each target version

More details:

• add a new extension point for registries:

<extension target="org.nuxeo.studio.core.StudioComponent"
point="registries">

<registry
modelClass="org.nuxeo.studio.client.features.schema.DocumentSchema"
readerClass="org.nuxeo.studio.core.readers.DocumentSchemaReader"
class="org.nuxeo.studio.client.registries.DocumentSchemaRegistry" />

</extension>

The model class refers to a ContributionModel instance, and other classes allow to generate readers and registry classes. They all have to follow appropriate interfaces. The reader and registry classes are not put alltogether as the reader only makes sense on the server side (registry can be useful both on client and server sides)

• make it possible to specify resources for a given target platform, for instance:

<platform id="dm-5.3.0">
<name>Nuxeo DM</name>
<version>5.3.0</version>
<types>org/nuxeo/studio/core/dm-53.json</types>
<operations>org/nuxeo/studio/core/operations.json</operations>
<help>OSGI-INF/help.properties</help>

<resource
modelClass="org.nuxeo.studio.client.features.schema.DocumentSchema"
value="data/platforms/dm-53.json" />

<builder>org.nuxeo.studio.core.builders.DocumentFeatureBuilder</builder>
...

resource can reference local files or remote ones (to get info from a live nuxeo for instance) but this will be done later.