[pulseaudio-discuss] desktop-notifications: Initial commit.

[Ștefan Săftescu]

This first set of patches adds the initial code for the desktop/ui notifications
GSoC project.

I have used Colin's and Tanu's suggestions for the previous patch to refactor
that code so that notifications are independent of back ends and back ends can
be written as modules and selected at runtime, through the notification manager.
The current implementation only supports one active back end at a time.

I have also moved the D-Bus code from the previous patch into a Desktop
Notifications back end that is not really complete yet; I need to:
    - receive D-Bus replies to get notification IDs
    - implement the cancel_notification function (requires the IDs)
    - add actions to notifications
    - register to the appropriate signals to receive user feedback
    
Let me know what you think.

Ștefan