display: Avoid unsetting the DESKTOP_STARTUP_ID variable too late

display: Avoid unsetting the DESKTOP_STARTUP_ID variable too late

Similar to what has been done recently for DESKTOP_AUTOSTART_ID [1],
we need to get rid of this call to g_unsetenv() in the displays'
backends for X11 and Wayland, so that it's guarantee to happen any
thread is created, while still being accessible when needed.

Let's stash the value of this environment variable when loading the
GDK library, and provide a private method so that it can be retrieved
from the displays' backend when implementing gdk_display_make_default().

[1] https://gitlab.gnome.org/GNOME/gtk/commit/22269902

Closes: https://gitlab.gnome.org/GNOME/gtk/issues/979