Rename document_t:{open,save} → document_t:sync_{open,save}

This makes it easier to search for code that use these functions.
Save All from find window uses default run-loop mode for saving

When saving and the document needs to execute a command (e.g. callback.document.export) then we can no longer use our special run-loop mode, as command execution is now using queues, and queue dispatching is paused when running in a custom run-loop mode.