Latte Dock Support

[mslazynski]

First, thanks for your work, birdtray is a great project solving a real issue in my everyday life. Is there any way to donate you?

Description

Having the unread count in the tray is very useful, but nowadays more and more people use the "macOS" desktop-style with a dock at the bottom of the screen. Given that each app has a separate icon in the dock, it's the best place to put some extra information, i.e. unread message indicator. It would be great if the birdtray was able to show the unread counter also on the dock, e.g. Latte Dock which is currently the most popular choice in the KDE community. There was a thunderbird extension doing just that, but It died due along with the XUL extensions. Still, it provides a code example (in Python) how it could be achieved: https://github.com/jamtur01/lattedock-thunderbird/blob/master/src/chrome/content/update-badge.py.

Example

There should be a new option in the settings allowing to display the unread count on the dock. It should available only if the supported dock is detected. When active, the unread counter should be displayed as a badge on the thunderbird icon in the dock.

[gyunaev]

This is fairly easy to do (Qt has dbus there), but since I don't use latte dock myself, I have no means to test it. You're welcome to submit a patch.