Replace DE open command in favor of xdg-open

[Jorge Villaseñor]

Talking with a friend of mine we realized that it could be a good idea
if pidgin/libpurple (does this must be front or back end issue?) uses
just xdg-open instead of asking if the user uses gnome or kde to open
URIs with their specific command.

Then, talking with darkrain about this issue, we noticed that there
are some code duplication regarding open files and URIs.

I was about to try to elimiate this code duplication but I want to ask
if there is some reason I'm not aware to not use xdg-open and
eliminate the DE specific commands and maybe the need of DE detection?
 AFAIK each DE is suppoused to set the xdg variables so xdg-open can
fetch the right program. It also help people using some DE different
from kde and gnome, or not using a DE (people who uses awesome).