Add a KDED module to ask for donations once a year

Reason for the change

KDE's recent experience with donations has been that when we ask, people donate. Our 2022 fundraiser brought in 35'000€, and the 2023-2024 one brought in over 140'000€. And these were advertised only through KDE's media channels, not in the software itself.

Income like this is important for KDE e.V.'s financial sustainability and ability to grow and expand initiatives, but it still pales in comparison to current and expected future expenses.

Other popular software-based nonprofits such as Thunderbird and Wikimedia ask for donations yearly in the UI of their software, and see huge returns from it. Accordingly, this commit implements a yearly system notification in Plasma that asks the user to make a donation. It's shown once in December, and then won't appear again for a year.

It's implemented as an on-by-default KDED module, meaning distros and users can permanently disable it using the normal means of disabling KDED modules (i.e. kcm_kded).

Test plan

1. Compile merge request with showInMonth changed to 7, and delete ~/.local/state/plasmashellstaterc if it exists
2. Restart KDED with systemctl restart --user plasma-kded.service
3. See a donation request notification

Screenshots or screen recordings

@teams/promo @teams/kde-ev-board