recreate plasmashellsurf on exposed, destoy on hidden

Summary:
every time the window gets shown the wayland surfaces
get created, when hidden, they get destroyed

showevent is too early for it as they aren't there yet,
use :Exposed instead, but only the first time when the surface
isn't there yet

Test Plan:
popup applets always have correct blur in a wayland
session

Reviewers: Plasma, Plasma on Wayland, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Frameworks, Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D5745