[Shell Corona] Don't show panels right away

ContainmentView tried to ensure the panel is only shown when it is ready but
the code didn't take into account the fact that UI ready is emitted asynchronously
so the panel would have never shown up leading to this client-side "workaround".

This requires 1d66098 of plasma-framework!

Differential Revision: https://phabricator.kde.org/D2475