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.