util: use async calls to show folder in file manager
Avoid blocking the UI if dzl_file_manage_show is called from the main thread if there were any issue with dbus.
To my eyes, this looks ugly, as dzl_file_manage_show is a sync call in three systems (Win32, OSX, and Linux). Ideally, we could have dzl_file_manage_show and dzl_file_manage_show_sync, or one async in all systems.
If the API user check error, still it will work for Win32 and OSX. For Linux, it will be managed by dzl_file_manage_show.
I welcome ideas to improve it.