Revert "hide "Update library" menu item if the current backend is not connectable"

This reverts commit 666fa7187ef70237fb0ce9ca55ed8cd5a20e5ff4.
factor out the checkBackendConnectivity filter to a class so it can be reused
use the backend connectivity filter for backend/updateLibrary, otherwise we'll
get a slightly cryptic error message instead of the friendly "the backend is
not connectable".