Call file manager launch API asynchronously

When launching a desktop icon, the launch API might block for a while (if the file manager puts up a dialog, for example). This will block xfdesktop's UI thread, and undesirable things (like tooltips not hiding) will occur.

Running asynchronously fixes that, however there's a possibly annoying side-effect: in some of these cases, it's actually probably preferable that these sorts of dialogs in effect act as modals and block user input on the desktop window. Now that behavior is gone. Ideally I should de-sensitize the icon view while this dbus call is running, but how things are currently set up makes that not trivial to do.

Closes #148 (closed)