[Feature] Messages notification

This pull request adds the building blocks for a general messages notification system. Its design complies with what have been discussed in the mailing lists. Also, adds the support for system messages notifications as it was discussed.

To try it out, simply open a Terminal activity instance, and run the following python script http://www.fpaste.org/82300/39623611/

To trigger notifications in regular intervals, for the scenario where the palette is already opened, you can use the following script http://www.fpaste.org/82303/62889139/