Add a portal helper

Instead of open-coding checks for sandboxing in multiple
places, add a helper function. This function is identical
to glib_should_use_portal() in gio.

gtk/gtkprivate.c

@@ -267,3 +267,23 @@ _gtk_ensure_resources (void)
 
   g_once (&register_resources_once, register_resources, NULL);
 }
+
+gboolean
+gtk_should_use_portal (void)
+{
+  const char *use_portal;
+  char *path;
+
+  path = g_strdup_printf ("%s/flatpak-info", g_get_user_runtime_dir ());
+  if (g_file_test (path, G_FILE_TEST_EXISTS))
+    use_portal = "1";
+  else
+    {
+      use_portal = g_getenv ("GTK_USE_PORTAL");
+      if (!use_portal)
+        use_portal = "";
+    }
+  g_free (path);
+
+  return g_str_equal (use_portal, "1");
+}

gtk/gtkprivate.h

@@ -95,6 +95,8 @@ gboolean        _gtk_propagate_captured_event  (GtkWidget       *widget,
 gdouble _gtk_get_slowdown ();
 void    _gtk_set_slowdown (gdouble slowdown_factor);
 
+gboolean gtk_should_use_portal (void);
+
 #ifdef G_OS_WIN32
 void _gtk_load_dll_with_libgtk3_manifest (const char *dllname);
 #endif