Integrate systemd notification support

[RodrigoRosmaninho]

This would be a direct port from @tuxPT 's repository.

In my opinion this should be disabled by default and enabled by the user either in the tutorial screen or later in a configuration UI. The user should also be able to indicate preferred times for the notifications to appear.

This may be achieved with the ~/.ementasrc json file that is already present. However, a mechanism to allow the user to later change his mind needs to be implemented.

(More people are welcome to assign this issue to themselves)

[tuxPT]

I suggest to make this repo a library and migrate @RodrigoRosmaninho repo to a terminal interface module and my repo to a notification module and then merge those repos in this repo