Pprz center tool update

tkdiff is no longer in the ubuntu repository, so when paparazzi center promts with the "changes were made" screen and you click the View Changes, nothing happens because most users who use a new version of ubuntu don't have ubuntu. They also don't know what the problem is because no feedback is given unless you run ./paparazzi from the commandline (people will usually create an icon to click). 

I propose meld, its clean.

Maybe this should even be a user setting.