#243 Standalone GLSP implementation

[tortmayr]

Resolves #243:

• Implementation of the Theia-independent GLSPWebsocketDiagramServer
• Refactoring of GLSPTheiaDiagramServer to use a common codebase
• server-side implementation for the WebsocketGLSPServer + minor refactoring
• Add workflow-standalone example for testing
• Add tasks for running the standalone example in chrome

To start the WebsocketGLSPServer for the workflow example simply add "websocket" as launch argument (java -jar workflow-example-1.2.0-SNAPSHOT-glsp.jar websocket)

(Note: Due to know issues it's currently not possible to use JSON-RPC with LSP4J and websockets out of the box (see #243). So for now the WebsocketDiagramServer does not use the JSON-RPC protocol and instead communicates with the server by simply sending action messages to a specific endpoint. This can be addressed in a follow-up PR once https://github.com/eclipse/lsp4j/pull/314 is merged)

[tortmayr]

Counterpart commit for glsp-p2: https://github.com/eclipsesource/glsp-p2/pull/3