Open containing folder doesn't work in flatpak build

The command "Open Containing Folder" does nothing when evince is run from flatpak. The function ev_window_cmd_open_containing_folder fails because inside the sandbox there is no default application associated to the file type "inode/directory."

One workaround might be to use g_app_info_launch_default_for_uri, which calls a flatpak portal to determine the appropriate handler. See the referenced merge request.