Improved Eclipse Che extensions development flow

[skabashnyuk]

The goal of this task is to provide up-to-date view how to build Eclipse Che extension. Before we can do that we would like to optimise couple of things.

1. We need to have strong definition of the things that user can override and that is not.
   Basically users don't have a choice to override Orion editor or docker. So it's not a plugin.
   And giving them ability to override any components may be risky. Move Orion Editor from plugin to the IDE Core module #4430

2. Setup of GWT compilation is quite long. https://github.com/eclipse/che/blob/871ef34f84ceb1a729faf1f12c8430e4e26d042f/assembly/assembly-ide-war/pom.xml#L345-L417 We agree with Florent's opinion that it would be nice
   to combine all of that in single maven plugin che-gwt-maven-plugin.
   The goal of this plugin is to provide any necessary actions for GWT compilation in single place.
   Also it should fail fast if some gwt components are missing with good error message. GWT error messages is quite hurd to read. Che-gwt-maven-plugin #4373

3. We also would like to make a two concept "Base" and "Default" assembly of agent-server and
   agent-client.
   "Basic" - means the minimum set of components necessary to run IDE and edit file.
   "Full" - means default functionality of Che or Condenvy.
   In this way users may decide: does they want to have something really small and simple or
   they want to extend che or Codenvy Basic ws-agent IDE and ws-agent server artifacts #4403

4. Add to @org.eclipse.che.ide.api.extensio.Extension parameter that will describe path to the gwt.xml file for corresponding module. By default if this parameter is not set try to use file with same name like annotated class but with extension gwt.xml. This allows us before GWT compilation phase check all inherits and throw exception if declared module not found in classpath. Module parameter in org.eclipse.che.ide.api.extension.Extension #4404

• Move plugin-dashboard to ide core Move plugin-dashboard to ide core #4614
• Move che-plugin-machine-ext-client to IDE core Move che-plugin-machine-ext-client to IDE core #4596
• Find a better place for classes from che-plugin-sdk-env-local Find a better place for classes from che-plugin-sdk-env-local #4595
• Make che-plugin-product-info optional for IDE core Make che-plugin-product-info optional for IDE core #4594
• Move into separate module OAuthTokenProvider from che-core-api-auth Move into separate module OAuthTokenProvider from che-core-api-auth #4462
• Plugin-docker is not a plugin Plugin-docker is not a plugin #4432
• Make plugin-factory part of ide core and wsagent Make plugin-factory part of ide core and wsagent #4619