Merged all demos into a single project

* The following demo-branches were merged:
** `demo--aggregate`
** `demo--aggregate--hopping-window`
** `demo--aggregate--hopping-window--suppressed`
** `demo--aggregate--session-window`
** `demo--aggregate--session-window--suppressed`
** `demo--aggregate--sliding-window`
** `demo--aggregate--sliding-window--suppressed`
** `demo--aggregate--tumbling-window`
** `demo--aggregate--tumbling-window--suppressed`
** `demo--stream-stream--inner-join`
** `demo--stream-stream--left-join`