Added ability to reject request actions

[spoenemann]

It should be possible to reject request actions. This change introduces RejectAction for this purpose.

This is particularly important for requests that are forwarded to a diagram server, e.g. RequestModelAction. We should implement the new feature in sprotty-server, too, after this is merged.

[gitpod-io]

[JanKoehnlein]

LGTM.

I wonder whether we should by default allow an error JSON object in addition to the message, similar to console.log()

[spoenemann]

I thought about that, too, but what type should we use? any?

[JanKoehnlein]

Not any, as these messages are likely to be serialized, but maybe something like in microsoft/TypeScript#1897 (comment)

[spoenemann]

I added a detail property to the new action and used the proposed JSON types where I found them appropriate. Please review.

[JanKoehnlein]

Thanks, LGTM!