applets/kicker: fix Plasma.dialog being inappropriately resized
Kicker reuses the current level's cascading popup when a new one is opened, rather than creatinga completely new one. This usually works out but fails if the submenu itself has a submenu - the submenu gets resized correctly, but afterwards is resized again to its previous size. The specific cause seems to be the subsubmenu hiding itself, if we instead wait until it is destroyed on the next event loop clear, the resizing doesn't happen.
So let's not hide it. This seems to be fast enough even on old computers to not lead to perceptible lingering subsubmenus, and even if it did happen in some rare situations, this would seem preferable to having a menu that's the wrong size long-term.
This doesn't feel like the cleanest solution, but after digging around for a substantial amount of time on Sunday, I couldn't figure out what exactly causes the resize event to happen on the submenu dialog, only that it doesn't happen when we skip the visibility=false. Given that Plasma.Dialog is on the way out, I guess it should be ok until Kicker is ported away from that.
Tested while building kwin on my almost 10-year old laptop.
For screenshots see the bug report.