[shell] Properly position DesktopView on Wayland

Summary:
On Wayland we need special glue code to make KWin aware of the position
the DesktopView wants to be. This change ensures that DesktopView calls
setPosition on the PlasmaShellSurface on creation and whenever the screen
geometry changes.

With this change DesktopViews are properly positioned in a multi-screen
setup on Wayland.

Reviewers: Plasma on Wayland

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D1889