wayland: Setup window management integration on initial commit

Originally, there was ShellClient class that was the predecessor to WaylandClient subclasses. If the main surface is unmapped, the corresponding instance of ShellClient would be kept around. That had been changed with the introduction of XdgToplevelClient and XdgPopupClient.

Since ShellClient had been kept alive even if the corresponding surface were unmapped, it did make sense to setup window management integration when the surface is mapped.

However, this also means that plasma will not know about initially minimized windows.

Since unmapped windows are handled differently now, we can setup window management after the client performs the initial commit. If the main surface is unmapped, the XdgToplevelClient object will be destroyed and with it, the window management integration.