gdkdisplay-wayland: Add API to set startup notification ID

gdkdisplay-wayland: Add API to set startup notification ID

For wayland clients, the startup notification ID is currently only set
from the DESKTOP_STARTUP_ID environment variable. As that variable is
only set for clients launched via exec(), startup completion is not
indicated correctly for DBus-activated applications unless an explicit
ID is specified - usually that is not the case, as the default handling
uses gdk_notify_startup_complete().
To address this, we need API to set the startup notification ID from GTK
as we have on X11.

https://bugzilla.gnome.org/show_bug.cgi?id=768531