Fix shifted keys leaking modifiers

[senekor]

closes #135

[senekor]

This is A PoC to discuss the general approach.

1. Shifted keys are not transformed into MultipleKeyCodes, but rather a ModifiedKeyCode. If this should be the default behavior going forward, the layout macro still needs to be adjusted accordingly.
2. State::keycode only reports the key code without any modifiers, preventing them from being associated with other key presses.
3. Layout::keycodes checks it the last state is a ModifiedKey, in which case the modifiers are added to the list. This ensures the modifiers are only sent if the modified key (e.g. &, +) was the most recently pressed key.