testing/xfce4-hamster-plugin: new aport

This is a recreation of the gnome-shell hamster extension as a xfce4 panel plugin.

Upstream is here: https://github.com/projecthamster

As a pet project of mine, I volunteer to maintain its and the hamster time keeping applications packaging for until I reach retirement and beyond. Both projects are feature-complete and moving at a mature pace of once or twice a year releases. Disruptive changes are only expected when python3 or gtk3 reach EOL.

I thought it would fit nicely to alpine since it seems to have a tendency towards xfce4 as its desktop.

This MR supersedes merge request !28950 "add: xfce4-hamster-plugin"