Remove keyboard_event.pressed from POSIX

[vladdeSV]

On Windows, a keyboard event can be in two states. Either "was pressed" or "was released". On POSIX however, a keyboard even is always pressed. There is no "key released" event emitted, only a "key pressed" event. This means that you cannot write a cross-platform terminal application which relies on key releases.

Should the struct KeyboardEvent instead be:

struct KeyboardEvent
{
    auto hasControlKey(SCK ck)
    {
        return controlKey.hasFlag(ck);
    }

    public SK key;
    public SCK controlKey;
    version(Windows) public bool pressed; // only applicable to Windows systems
}

[vladdeSV]

Unittests would be have to be written per OS, since pressed does not exist with POSIX.