Code Health: Update existing winrt properties to use `GETSET_PROPERTY` when possible

Pretty self explanatory. Classes with a bunch of

    uint32_t TerminalSettings::DefaultForeground()
    {
        return _defaultForeground;
    }

    void TerminalSettings::DefaultForeground(uint32_t value)
    {
        _defaultForeground = value;
    }

Is an enormous amount of boilerplate we don't need.

GETSET_PROPERTY(uint32_t, DefaultForeground, DEFAULT_FOREGROUND_WITH_ALPHA);

is better.