Suppress "could not create KWindowShadow" warnings in plasmashell

Plasma::Dialog can attempt to create a window shadow before the window becomes visible on the screen. This is a legit case but WindowShadow will print a warning if that happens.

This change rearranges the creation logic a bit. If the window already has a surface, KWindowShadow::create() will succeed. If the window does not have a surface, an event filter will be installed and the shadow is going to be created when the window is exposed. On the other hand, if the shadow manager global is not present, KWindowShadow::create() will return false.