Add Mac Notification Plugin

[kkonsw]

This PR adds an initial implementation of plugin that allows to display notifications on MacOS. It has a dependency on terminal-notifier: https://github.com/julienXX/terminal-notifier

Install terminal-notifier with Brew:

brew install terminal-notifier

On my system I also had to allow notifications from terminal-notifier in System Settings.

Build Dino with the mac-notification plugin enabled to receive notifications:

./configure --enable-plugin=mac-notification

Currently the following notifications are displayed:

• Message Notification
• Call Notification
• New Contact Notification
• New MUC Invite Notification