display: Ensure ::x11-display-opened is emitted at a time there are handlers
During display initialization, the MetaX11Display would be opened at a time when we didn't initialize yet the compositor/plugin. The plugin is certainly interested in ::x11-display-opened happening, so make sure the signal happens late enough that the upper layers are able to connect to the signal in advance.
Fixes xsettings/ibus-x11 initialization on the still default "initialize Xwayland on startup" wayland compositor behavior.
Closes: #771 (closed)