Improvement/58/feature gate font kit

[azriel91]

Closes #58.

I thought iced wasn't working when nothing was showing up in the examples, and it happened to be #199 / #270 / #351. Should've searched issues earlier.

Successor to #364 -- couldn't reopen it after deleting the branch unfortunately.

I'm not sure whether to choose - or _ between feature words, since there's a "glow_canvas" feature in the top level Cargo.toml.

I've tested this using the custom_widget example for both iced_wgpu and iced_glow. For iced_glow I disabled default features from iced in examples/custom_widget/Cargo.toml, and then running:

cargo run --package custom_widget --no-default-features --features "glow"
cargo run --package custom_widget --no-default-features --features "glow glow_default_system_font"

[hecrj]

Looks great! This is exactly what I had in mind 😄

Just a couple of details and we can merge it.

[hecrj]

Looks great. Thank you!