Add Registry::interfacesAnnounced signal

Emitted when the Wayland display is done flushing the initial interface
callbacks, announcing wl_display properties. This can be used to compress
events. Note that this signal is emitted only after announcing interfaces,
such as outputs, but not after receiving callbacks of interface properties,
such as the output's geometry, modes, etc..

This signal is emitted from the wl_display_sync callback.

REVIEW:120471