Add a cake icon for birthdays events -- to quickly differentiate those kind of events (auto created) from others (manually created)

[brunetton]

Checklist

• [X] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• [X] I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• [X] I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• [X] I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• [X] This issue contains only one feature request.
• [X] I have read and understood the contribution guidelines.
• [X] I optionally donated to support the Fossify mission.

Feature description

First of all, I must say that I really enjoy this amazing piece of software, and I'm using it everyday :heart:

I would find it very convenient to be able to quickly differentiate birthdays events from other events in my calendar, as they are special.

I propose to add a glyph to visually make them visually identifiable; like Google's calendar:

What do you think about ? Would you be open to a PR ?

Why do you want this feature?

I always unfortunately miss birthdays in my events, as I've often more than a dozen of events everyday. I'd be happy to be able to quickly differentiate them and keep my friends ;)

Additional information

Fossify Calendar v1.0.3

Somewhat related to #97