it's already defined in configure.ac for all applets

netspeed.c:477:15: warning: variable ‘state’ set but not used [-Wunused-but-set-variable]
  477 |  GtkStateType state;
      |               ^~~~~
netspeed.c:476:15: warning: variable ‘ra’ set but not used [-Wunused-but-set-variable]
  476 |  GdkRectangle ra;
      |               ^~

void
g_list_free_full (GList *list,
                  GDestroyNotify free_func);

Convenience method, which frees all the memory used
by a GList, and calls free_func on every element's
data.

https://developer.gnome.org/glib/stable/glib-Doubly-Linked-Lists.html#g-list-free-full

Note that GTK+ sets a default title of _("About %s") on the dialog
window (where %s is replaced by the name of the application, but
in order to ensure proper translation of the title, applications
should set the title property explicitly when constructing a
GtkAboutDialog

https://developer.gnome.org/gtk3/stable/GtkAboutDialog.html

This removes old icons included with the applet and instead uses icons from the current theme.

Known issues:
* Loopback interface does not have an icon, instead we use a "reload" icon to simulate it.
* PLIP/SLIP interfaces do not have proper icons, I managed to find an old PLIP icon in the hicolor theme and re-purposed it.
* Since not all icons have the same levels of scaling, I'm capping the size at 48px to keep all interfaces consistent.
* There are no icons for Wireless Quality, so I retained the existing ones for now.

This improves support for HiDPI by loading properly scaled surfaces for applets.