wayland: Make mapping from xinerama indices to Output correct
We assume that outputs in kwinApp()->platform()->enabledOutputs() are stored in the xinerama order. However, this is not the case on Wayland and it's not going to be changed because it increases the complexity. This change makes Workspace::xineramaIndexToOutput() use Xinerama extension API to map a xinerama index to the associated Output object. With this, Xwayland applications will be able to put on outputs as expected. Note that xinerama indices are not cached because Workspace::xineramaIndexToOutput() is not used in any hot code path. If that changes, xinerama indices can be cached. The cache must be invalidated whenever we get screens changed notify event from RANDR.