Port to GAction and libpanel-applet 3.14

Currently workrave does not build against libpanel-applet 3.14, which has changed its API. This pull request attempts to fix it.

Note that I never used Workrave, so I have tested it only roughly (I made sure it builds, shows in the panel, and menu actions do something). In particular, I am not sure that radio actions work fine, as they are always disabled for me.

References:

- https://wiki.gnome.org/HowDoI/GAction
- https://wiki.gnome.org/HowDoI/ApplicationMenu
- [gnome-panel 3.14 announce](https://mail.gnome.org/archives/gnome-announce-list/2014-October/msg00036.html)