Standalone GLSP Client

[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 (https://github.com/eclipse/lsp4j/issues/210). 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 changed in follow-up PRs)

[CLAassistant]

All committers have signed the CLA.