#317 Linux version assumes nautilus as filemanager

[Vasco]

Problem:
When clicking the button "Open atlas folder" Mobac tries to use nautilus to open that folder (on linux). Nautilus is not always installed.

Solution:
Use xdg-open instead, that will launch the default filemanager.

[r_x]

nautilus is used in case Gome is detected as desktop, otherwise xdg-open is used.
Isn't nautilus the default file manager of gnome?

[Vasco]

Often it is caja or nemo nowadays (depending on gnome 2 or 3). xdg-open will work allways,

[r_x]

OK, changed it to use alyways xdg-open on Linux.