upload: plugin android gallery, first draft: - added "external" key in PLUGIN_INFO which indicate that a plugin doesn't create a widget when set to True - callback and cancel_cb method in UploadMenu now use a cleaning_cb arguments which can be None. If set, the callback will be called without argument once the file is uploaded - the AndroidGallery plugin looks for images on Android, then save the content to a temporary file an upload it, then clean the file. This plugin is only active on android platform.