screenpool: don't race with orderwatcher

depending on who gets signalled first the internal members of the watcher may present an earlier state of screen setups. force a refresh before pulling data out of the order watcher.

this resolves two problems I'm consistently experience when disconnecting my only enabled main screen:

• the shellcorona fails to match up desktops with screens in screenInvariants
• the screenpool's screenInvariants fails assertions when its screen list races with the one inside orderwatcher