Extended external communication via D-BUS

[leukipp]

External communication with cortile was implemented via unix domain sockets as discussed here #14 (comment).

The current implementation has some caveats, as it works in an asynchronous way. You have to listen for return values while you are requesting a state from cortile.
There were also feature requests that were either impossible to add or require extensive programming skills and knowledge of the X window backend.

To address these issues, I have opted to introduce a more efficient approach that uses quasi-synchronous interaction with cortile via D-BUS. Given that tray icon communication already relies on D-BUS, this change introduces no new dependencies.

For those who have developed custom event handlers/hook-up logic for cortile, I recommend transitioning to the new API as soon as possible.
The forthcoming external communication via D-BUS will supersede the now deprecated unix domain socket approach.

Details regarding the new D-BUS API will be published here and in the README.md file.