gksuexec should allow to select software through a menu

[Lucas Nussbaum]

The use case "Joe wants to edit a configuration file as root" or, more general
"Joe wants to run software as root", is currently hard to deal with.

When running applications with Applications -> Run application..., user can
select an application in a list of "known applications" (whose user-friendliness
could be improved, but that's not the point).

But when running applications with Applications -> System tools -> Run as
different user, the user has to know the executable name, which isn't very user
friendly.

Possible solutions :
- add the sudo feature to Applications -> Run applications (my favorite
solution, even if it is bloated)
- add the software list to gksuexec (causes duplication of code - not really
justified except from a development POV)