Use a gresource to store the css

This makes build / test faster since one can skip install. It's
also more obvious which file is being used.

src/server-context-service.c

@@ -338,9 +338,9 @@ update_widget (ServerContextService *context)
         context->widget = NULL;
     }
 
-    theme_path = g_build_filename (THEMESDIR, "default.css", NULL);
-
-    theme = eek_theme_new (theme_path, NULL, NULL);
+    theme = eek_theme_new ("resource:///sm/puri/squeekboard/style.css",
+                           NULL,
+                           NULL);
     g_free (theme_path);
 
     keyboard = eekboard_context_service_get_keyboard (EEKBOARD_CONTEXT_SERVICE(context));