Support creation of PlasmaShellSurface prior to ShellSurface

Summary:
So far KWin did not support the sequence:
1. Create wl_shell
2. Create PlasmaShellSurface
3. Create wl_shell_surface

KWin only supported the case that the PlasmaShellSurface is the last
thing to get created. This is rather limiting and can be considered a
bug. At least we had a QEXPECT_FAIL auto test for this situation. So
it was a known issue.

This change should make it easier to support the QtWayland changes in
Qt 5.8.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4482