Keyboard modifiers rework

[ua-kxie]

• As I've brought up in keyboard modifiers available alongside events #1868, keyboard modifiers should be made available at a higher level in the Event struct, to allow pattern matching on things like ctrl-click.

• Currently pattern matching on no modifiers can be done in the following way:

const NO_MODIFIER: Modifiers = Modifiers::empty();
match event {
    Event::Keyboard(iced::keyboard::Event::KeyPressed {
        key_code: iced::keyboard::KeyCode::C,
        modifiers: NO_MODIFIER,
    }) => {
        ...
    }
    Event::Keyboard(iced::keyboard::Event::KeyPressed {
        key_code: iced::keyboard::KeyCode::C,
        modifiers: Modifiers::CTRL,
    }) => {
        ...
    }
    ...
}

While this works, there is still some problems. Aside from feeling awkward, I've noticed that the code still compiles if the const definition const NO_MODIFIER: Modifiers = Modifiers::empty(); is omitted; the pattern matching against NO_MODIFIER is interpreted as destructuring instead.

I've noticed that a number of newer libraries have popped up claiming to address problems with the bitflags crate. Perhaps one of these is worth considering?