Open default application using async function

[yetist]

Before this patch: press F1, yelp show the gnome desktop help documents press WWW, browser show a blank page.

After this patch: press F1, yelp show the mate desktop help documents press WWW, browser show the mate-desktop website.

Open default application using async function

[cwendling]

Not tested, but:

• opening up the mate-user-guide when opening up yelp makes sense to me, indeed especially as yelp's default is GNOME's user guide. Beware though that this could potentially be missing on the user's computer (I myself don't have it, I don't need it and it takes some HDD space), and yelp isn't super welcoming when requesting a missing section.
• I however really don't like opening a specific web page using the "launch web browser" keyboard shortcut: IMO, this should just open the browser, not make an unrequested query to any wbesite (be it the MATE one or anything else).

The biggest difference between the two is that the Yelp case is local, and the WWW browser one is remote.