make shadows work on wayland

Summary:
since from Qt 5.8 QtWayland destroys its surfaces every time
a window gets hidden and recreates them again when is shown
(that's how the protocol is defined) install the shadows
every time the window is shown, using a map to keep track of surfaces, in order to delete them on window hide and avoid leaks

Test Plan: popup menus have correct shadows on wayland now

Reviewers: #plasma, hpereiradacosta

Subscribers: anthonyfieroni, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5910