Make web view fully asynchronous

[snomos]

The server client connection Is asynchronous, but the display is not. This means that the user has to wait for all responses to arrive before anything is displayed. This can take a long time for long documents.

Instead the grammar checker sidebar should update its view as the responses arrive. Some sort of feedback must be visible informing the user whether the grammar checking is finished or still pending, with an option to abort/stop the remaining checks.

If the user clicks "Check" to check the document anew before the old request is done, it should be automatically cancelled.

[snomos]

We agreed that it is labour intesive (and thus expensive) to make the backend requests asyncronous at this time, so we should just show a spinner to let the user know that the plugin is checking the grammar.

Both GDocs and MS Word have spinners now (cf #49), so we can close this issue as won't fix.