Both Archlinux and Fedora 29 have guidelines to point explicitly at
/usr/bin/python2 where it expect python2.
Also, do not use env.
FixesQubesOS/qubes-issues#4027
Since we have proper python package, use it instead of hacky one-file
package. This will ease installation and packaging, including switching
to python3.
Instead of directly using Exec= line, parse the file (at the launch
time) with Gio library. The main reason for this change is to handle
Terminal= option, but generally this approach should be more
bulletproof, especially when some fancy options are present in desktop
files.
