Draw diagram requests should interrupt/cancel old requests

[NiklasRentzCAU]

Every diagram request sent to the diagram server will currently be processed and a new diagram will be sent for each request. When new multiple requests arrive at the same time, they will get blocked by the synchronization, but eventually finish. I propose a new process for this:

• A single diagram request should always be handled and completed.
• A second diagram request (for the same diagram widget) should wait until the first one has finished.
• Any other further diagram request should cancel all previously waiting diagram requests (as they will not be needed anymore), thus being the only waiting request.

This will prevent issues currently happening for example during the simulation of SCCharts, where the current values are added to the edges in the diagram: A rapidly ticking simulation will cause a long queue of diagram requests to be stuttered off over time.