qt: use KWindowEffects::enableBlurBehind() and enable blur in compositor_wayland

On X11, the blur effect availability can be checked through _KDE_NET_WM_BLUR_BEHIND_REGION_NAME atom. Although messy, it seems to work fine.

On Wayland and other platforms, the effect availability can be checked using KWin D-Bus interface, but I could not find a way to enable an effect on a specific window. This merge request seems to be a better approach than to depend on KWin directly. In the future, KWindowEffects may even support Windows.

One can also use ENABLE_BLUR_BEHIND_HINT window property as in !3150 (closed), but that requires the plasma-integration/platformtheme to be loaded as Qt Platform Theme. Since it is necessary to know if the effect is available, I went with this approach.