gdk: Add a private api to find sandboxes

This will be used in more places in the future.

gdk/gdk-private.h

@@ -77,4 +77,6 @@ typedef struct {
 GDK_AVAILABLE_IN_ALL
 GdkPrivateVTable *      gdk__private__  (void);
 
+gboolean gdk_running_in_sandbox (void);
+
 #endif /* __GDK__PRIVATE_H__ */

gdk/gdk.c

@@ -468,6 +468,19 @@ gdk_display_open_default (void)
   return display;
 }
 
+gboolean
+gdk_running_in_sandbox (void)
+{
+  char *path;
+  gboolean ret;
+
+  path = g_build_filename (g_get_user_runtime_dir (), "flatpak-info", NULL);
+  ret = g_file_test (path, G_FILE_TEST_EXISTS);
+  g_free (path);
+
+  return ret;
+}
+
 /**
  * gdk_display_open_default_libgtk_only:
  *
@@ -601,7 +614,6 @@ gdk_init (int *argc, char ***argv)
  * management for you.
  */
 
-
 /**
  * gdk_threads_enter:
  *