iconGrid: Subclass a C actor for BaseIcon

We create a lot of BaseIcons for the appGrid, one for every app, and for
all of those we have to hop through JS to get the preferred width. That
makes it another obvious target for moving to C, so let's do that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1755>

src/shell-square-bin.c

@@ -0,0 +1,43 @@
+#include "config.h"
+
+#include "shell-square-bin.h"
+
+struct _ShellSquareBin
+{
+  /*< private >*/
+  StBin parent_instance;
+};
+
+G_DEFINE_TYPE (ShellSquareBin, shell_square_bin, ST_TYPE_BIN);
+
+static void
+shell_square_bin_get_preferred_width (ClutterActor *actor,
+                                      float         for_height,
+                                      float        *min_width_p,
+                                      float        *natural_width_p)
+{
+  float min_width, nat_width;
+
+  /* Return the actual height to keep the squared aspect */
+  clutter_actor_get_preferred_height (actor, -1,
+                                      &min_width, &nat_width);
+
+  if (min_width_p)
+    *min_width_p = min_width;
+
+  if (natural_width_p)
+    *natural_width_p = nat_width;
+}
+
+static void
+shell_square_bin_init (ShellSquareBin *self)
+{
+}
+
+static void
+shell_square_bin_class_init (ShellSquareBinClass *klass)
+{
+  ClutterActorClass *actor_class = CLUTTER_ACTOR_CLASS (klass);
+
+  actor_class->get_preferred_width = shell_square_bin_get_preferred_width;
+}