effects/overview: Hide panels
Since panels are visible currently, you can be tricked into thinking that they are interactive, but they are not due to various technical details.
With the current design of plasma, the compositor and the desktop shell live in two different processes. In theory, it will result in a more robust desktop; in practice, implementing features such as interactive panels in the overview effect become difficult.
On Wayland, panel interactivity issues are solvable, but we would still have other issues, e.g. panel popups not being animated as during normal operation mode, etc.
For the time being, this change makes the overview effect hide panels. How to solve the panel interactivity problem is TBD.