Improve read only performance when just displaying rich workspace

[juliusknorr]

In most cases the rich workspace area is probably not edited, so we should think about having a more sane process for opening up a read only editor and just kicking of the heavy sync parts and session setup when starting to edit.

[max-nextcloud]

I profiled the workspace loading some and it turns out rendering the menubar is surprisingly expensive.
I experimented some with it and we can postpone that until the content has been rendered. In theory this would reduce the time till the content is displayed by 600-800ms. In practice it's still hard to measure.

[ImaCrea]

Great idea :) It's very disturbing when I'm ready to click on a file and then everything scrolls down because finally the editor is loaded. At least having a placeholder before it's loaded so elements doesn't move when editor is loaded would be an improvement.

[tcitworld]

The PR from @max-nextcloud doesn't really answer the original issue, may this issue be reopened to avoid loading sync & session until the workspace is actually opened (to avoid make the backend work for nothing) ?

[juliusknorr]

#4776 also contributed to that by showing the content already before the session is opened but what would be missing is a read only and just open the session once clicked option