Improvement: GTK: Utilize appindicator to create a notification icon for Ubuntu

Blather needs a notification icon that contains:
- a "Listen" option which works off the Listen button state.
- a "Continuous Listen" checkbox which, when activated, disables the "Listen" option and sets Blather to continuous mode. When unchecked it should stop Blather from listening and enable the "Listen" option.
- an Exit option.

The notification area icon should be icon.png under the data directory.

In the future we may add additional code to detect OS type and use other notification area classes, but for now we can focus on Ubuntu with appindicator.