Publish modifier flags from event

[eirnym]

Event should expose modifier flags as NSUInteger or parsed.

Parsed version could be a bitmask wrapper with custom methods like this:

struct ModifierFlags(pub NSUInteger);

impl ModifierFlags {
    fn raw(&self) -> NSUInteger { self.0 }
	fn capsLock(&self) -> { self.0 & CAPS_LOCK_BIT }
    // ...
}

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