Don't touch exe line in .desktop files

How to reproduce:
Create a .desktop file with this Exec property with an editor and NOT with exo-desktop-item-edit:

bash -c "`basename -s .desktop \\\`xdg-mime query default audio/mpeg\\\`` ."

When we load and save this .desktopfile with exo-desktop-item-edit then the number of backslashes will change which shouldn't be the case. If you wonder what the command is doing, we simply call the default audio player and pass the current directory as first argument.

Solution:
In GLib there are getter and setter methods for GKeyFile which ignore escape sequences. For example check this doc.

The patch only applies this for the Exec property. Should we do this also for other .desktop file properties?