GLSP-1240: Ensure compatibility with >= Theia 1.44

[tortmayr]

Fix API breaks introduced with Theia 1.44.
Update Theia Version Compatibility section in README.md.
Note that mentioned 2.1.0-theia1.44.0 version is not released yet.
I will do so once this change is approved.

[CamilleLetavernier]

Looks good to me! I used this for #197 and it worked fine.

Note: I didn't do extensive testing of the entire GLSP stack, so another review is still welcome!

[ndoschek]

Thanks, LGTM too 👍

[tortmayr]

In general the change seems to work great and interrupt/reconnect works for the most cases.
However, there seems to be an issue if the server sends actions/updates while the frontend is disconnected.
Those actions just get lost and never reach the frontend client.

I tested this by updating the server to send a MessageAction with an incremented counter each second.
If I disconnect the frontend for 10 seconds those intermediate message updates get lost:

This becomes more of an issue if the frontend disconnects while a RequestAction or any other promise that needs a server action to resolve is dispatched. E.g. if the frontend disconnects during model loading and reconnects afterwards the model is never loaded.

Theia has supports buffering of messages (see https://github.com/eclipse-theia/theia/blob/3034c0a0b4dd1082589ee5ec78a066a8532705fa/packages/core/src/node/messaging/websocket-frontend-connection-service.ts#L129) while the socket is disconnected so I think some special handling is required on our side to properly forward those messages to the glsp client.