window: expose outputs to applications

window: expose outputs to applications

Add output_configure_handler as a display property. This exposes only
configured outputs, that is the current mode info is already received,
to applications.

The handler is also called for mode changes on an existing output. This
simplifies the implementation in toytoolkit as we can defer the handler
calls from wl_output binding time to when we receive the current mode.
We do not need separate handlers for "new output" and "mode changed". A
plain "new output" handler would be problematic as the current mode is
not known yet.

Also add delete_handler hook for outputs, but that will never be called
for now, as the protocol lacks a way to signal output disconnections.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>