Check if system is using a dark theme

[d.healey]

Now that we have CSS I think it's quite likely we'll start to see developers wanting to provide both light and dark theme variations, with the ability to select the correct one based on the user's system theme.

So I've added an isDarkMode property to the output of Engine.getSystemStats().

I notice JUCE also provides a way to add a listener for this so you could get a callback if the system theme is changed. @Christoph-Hart That would be a good candidate for the broadcaster system perhaps?