feat: add feature for load system fonts according user input

[lj94093]

Solved the issure mentioned in #1344.

For now,this PR adds a system_fonts feature which depend the font-kits and skia-safe(gl feature) and it makes the binary size very big.So I add it as a optional feature in epaint ,egui and eframe for some people who need it.

Maybe I should develop a library to choose the font for characters.But it sounds like there are lots of work to do.If i finished it or find a proper library,i will make a PR again add as the default feature of eframe.So for now,Let's just make it as a experimental feature.

[emilk]

Hi, and thanks for the PR!

Can you describe what this is supposed to do? The new code is very sparse on documentation.

I ran it on my mac and pasted some Japanese characters into a text box. The app froze for a second, and then the familiar ◻◻◻ replacement characters showed up. Maybe it doesn't work on Mac?

epaint and egui are designed to be completely disconnected from the platform they are running on, and this PR breaks this tradition. I think we should think about whether we can design this differently. For instance, this could be a plugin system where you can tell epaint "use this callback to look for fonts for missing characters". That way we could use the same plugin system to also implement this on web (some sunny day).

[lj94093]

Sorry for didn't test it on mac os.I tested it on my Windows.

The correct way for use this feature is add system_fonts in project's Cargo.toml.

The reason for it didn't load correct fonts on mac is that font-kits's family name didn't match with the name in skia.

Luckily,I find that skia-safe already load the font data from disk.So we don't need to load fonts by font-kits .So we can decrease the font loading time and remove the font-kits dependency.

I fix it in later 2 commits.

Reason for didn't implement it with a plugin system:
The skia-safe is too big for egui.If we implement a plugin system for load system fonts with this feature opened,the binary size of egui will be very large.It is not suitable for such a lightweight GUI framework.

If I find a better way to to choose the font for characters and make the binary size small enough.Then we can implement such a plugin system.