bitcoin-qt built with CMake is lacking support for desktop
notifications.
This diff adds DBus support to bitcoin-qt and enable the feature.

Build and run on all platforms.
Check the notifications are popping.

How to make a notification to appear:

• Start bitcoin-qt with the regtest option.
• Open the console window.
• Generate a block with generate 1

Tested working on Gnome shell, OSX (Sierra, I didn't manage to make it
work on Catalina ?) and Windows 10.