The pad widget was partially updated to the new views, but was no longer
working. In this commit, we rewrite the widget to make it work.
Note that we did some functional changes:
- we do only one rpc to check if the server is configured. the result is
stored on the prototype, so we do not need to do it again
- we do some rpcs with shadow: true, so we will not stop the rest of the
ui if the server is slow to respond
- we display a loading message in readonly, when the content is not
loaded yet.