Fix signalToIcon() to match GNOME Settings WiFi icons
Fixes a visual bug where GNOME Settings and GNOME Shell would sometime display different icons for the same wifi network by using the same signal thresholds (see https://gitlab.gnome.org/GNOME/gnome-control-center/-/blob/master/panels/network/cc-wifi-connection-row.c#L301)
Fixes #217 (closed).
Edited by Remi Salmon