Implement CONFIG_KEY_UNSIGNED_INTEGER

strtol() does not work when trying to assign 32 bits of data into a
regular signed int on 32 bit systems. Use corresponding strtoul()
instead.
diff --git a/clients/desktop-shell.c b/clients/desktop-shell.c
index 4848449..3d5119a 100644
--- a/clients/desktop-shell.c
+++ b/clients/desktop-shell.c
@@ -107,8 +107,8 @@
 static const struct config_key shell_config_keys[] = {
 	{ "background-image", CONFIG_KEY_STRING, &key_background_image },
 	{ "background-type", CONFIG_KEY_STRING, &key_background_type },
-	{ "panel-color", CONFIG_KEY_INTEGER, &key_panel_color },
-	{ "background-color", CONFIG_KEY_INTEGER, &key_background_color },
+	{ "panel-color", CONFIG_KEY_UNSIGNED_INTEGER, &key_panel_color },
+	{ "background-color", CONFIG_KEY_UNSIGNED_INTEGER, &key_background_color },
 	{ "locking", CONFIG_KEY_BOOLEAN, &key_locking },
 };