Remove the shebang line in paperwork.desktop

Other applications do not have it. Moreover it makes debian’s lintian
trigger a warning.
Merge pull request #322 from plietar/remove-desktop-shebang

Remove the shebang line in paperwork.desktop
Backend: Don’t raise an exception if saving the config fails.
Frontend: exit nicely on SIGINT and SIGTERM.

Using SIG_DFL kills the process immediately, not saving the configuration.
Proper handling is required.
Merge pull request #323 from plietar/fix-signal

Improve configuration saving
Fix: older versions of GLib don't have the method "unix_signal_add" available

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Merge branch 'testing' into unstable