New accessible event style in calendar grid

[GVodyanov]

Fix #8022
Fix #2353

How do we like this? It passes WCAG AA with all colors ✅. Based on this design: #8022 (comment)

Here's what it looks like with some "normal" colors:

	Yellow	Purple
Light mode
Dark mode

Here's what it looks like with the most problematic colors, still passes WCAG:

	White	Black
Light mode
Dark mode

Naturally this system doesn't look great with people choosing white for their white backgrounds, but who am I to stop them from doing that (also our current system doesn't really handle that either). It passes WCAG anyway.

[SebastianKrupinski]

Looks great!

[kra-mo]

Looks really nice :)

My only question is, does it pass WCAG AAA for high contrast? If it does, then approved.

[GVodyanov]

Added handling specifically for high contrast mode,

I removed transparency from the events because if not the VERY contrasting calendar grid was making the event title hard to read

[GVodyanov]

Also at @ChristophWurst's request added a little icon to see if there are multiple people invited to the event

[SebastianKrupinski]

Since we are working on event styles, can we maybe add this to the PR

#2353

[GVodyanov]

Since we are working on event styles, can we maybe add this to the PR

#2353

Sure yeah why not, but I need a plan from @kra-mo or @nimishavijay first pls. Here is how outlook currently does it:

Should we also do this?

[kra-mo]

Makes sense to me :)

[kra-mo]

This thread (for a different project) might also be of interest, quite a bit of research and experimentation there: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1433

[GVodyanov]

Here is Free/Busy!