Port to GAction and libpanel-applet 3.14

Currently, uim does not build with gnome-panel (and libpanel-applet) 3.14, as the library API has changed significally. This PR attempts to fix it.

Note that I am not a user of uim and thus have not tested it properly.

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)