This is especially important for qubes-desktop-run used inside DispVM.
The DesktopAppInfo.launch() method returns after just launching the
application. In DispVM case it worked by a coincidence - because the
launched application was keeping stdin/out open, which also prevented
DispVM killing. Use DesktopAppInfo.launch_uris_as_manager which at least
allows to learn PIDs of spawned processes, to track them manually.
This still doesn't fix gnome-terminal issue, or any other application
using either DBus activation, or any other client-server model. But at
least fix basic apps like firefox and xterm.
FixesQubesOS/qubes-issues#3213
Since we have proper python package, use it instead of hacky one-file
package. This will ease installation and packaging, including switching
to python3.
