Specify example feature requirements in a standard way.

[xStrom]

This PR adds the various example feature requirements to Cargo.toml via the required-features field.

This allows the examples themselves to be cleaned up from all the cfg guards, which add needless confusion for new developers.

$ cargo run --example list
error: target `list` in package `druid` requires the features: `im`
Consider enabling them by passing, e.g., `--features="im"`

[jneem]

Looks much better!