Add CONTRIBUTORS and LICENSE to source distribution
Use icon names rather than STOCK items

The corresponding stock items have been deprecated in GTK 3.10. Use the
icon names recommended by

https://developer.gnome.org/gtk3/stable/gtk3-Stock-Items.html
Separate Icon loader utility
Use Icon loader utility for TrayIcon class
Support choosing from multiple icon names
Use Gio.ThemedIcon

This provides fallback "broken image" icons if the icon is not found and
is much simpler to use.
Add some fallback icon names
Read icon names from config file
Add example for icon customization in config file
Revert "Relicense under GPLv2"

This reverts commit edb6d80270d372495981d58dd86c98508c1de238.

No need to migrate to GPL for now, since I do not plan to include a
bunch of considered icons from GPL licensed themes anymore.
Merge branch 'choose-icons'