DesktopFileManager::resolve_app_id should not return an app id with a .desktop file suffix

[mattkae]

Problem

The app_id that is returned by ForeignTopLevelManagerV1 typically ends with a .desktop suffix as it currently stands. We did this originally because g_app_info_get_id typically returns the app id this way, although it doesn't have to. In cases where the app id is not found with g_app_info_get_id, we were appending .desktop. We shouldn't do this, as this isn't necessarily correct.