Update to egui 0.21.0

[emilk]

This PR updates to egui 0.21.0

I would greatly appreciate if you could merge this and publish a new version of egui_dock as soon as you can 🙏

This also adds a small new feature: TabViewer::on_tab_button

[emilk]

This is finally ready for review @Adanos020

[emilk]

@Adanos020 usually a user would have:

egui = "0.21"
egui_dock = "0.4"

and it would just work, since the egui = "0.21 line would pull in default_fonts (as would any use of eframe).

This change is really only for the users that are explicitly opting out of default_fonts (because they are installing their own fonts):

egui = { version = "0.21", default-features = false }
egui_dock = "0.4"

---

I would suspect it to be extremely rare with any users that don't have either egui or eframe in the dependencies, but rely only on egui_dock and use egui_dock::egui when they want to use egui directly. For those users, they would need to add egui = "0.21" to their Cargo.toml to get the default_fonts back.

[Adanos020]

@emilk Then before publishing 0.4, I'd add a note about that in the Readme. I'll merge this PR for now and do that in a separate one.