kded: add explicit waiting time before setting colors

modulesChanged signal will take some time to reach a GTK3 app, so wait a
moment before "colorreload-gtk-module" is finally loaded into GTK3 apps.
This makes sure color schemes in GTK3 apps are updated as expected.

This may not be a problem after https://gitlab.gnome.org/GNOME/gtk/-/commit/a40126e1f91a0cc8c45203d9ff229f65952eb541
is merged, but if the module list does not contain the expected module,
users will still notice color schemes are not changed in GTK3 apps for
the first time users change the system color scheme.

CCBUG: 421745


(cherry picked from commit 90b42342)