Qt: Re-enable Dark Mode support

[spycrab]

The Qt version we use now should support Dark Mode properly.

[8times9]

The toolbar text now displays correctly in dark mode

[spycrab]

@8times9 Does the rest of the interface look okay too? (Especially TAS and the Controller config Windows for Wiimotes / Gamecube Controllers)

[JosJuice]

This only affects macOS, right?

[spycrab]

Yes

[MayImilae]

Here are a bunch of screenshots!

pr8640-Screenshots.zip

So two main issues I spotted. The C stick is black against dark grey, with only a little tiny bit of yellow peeking through. That's not great. For all other joysticks, the calibration bounds for the joystick, the dashed lines, are black against dark grey as well, and nearly invisible. Other than those two things, everything seems good.

[spycrab]

@MayImilae It seems like that code already uses palettes instead of hard coded colors, so fixing this will be a bit more difficult. Would you prefer this PR be on hold until we fix that or should we go ahead and merge it anyway?

[MayImilae]

I believe billiard is already looking into the indicators? Maybe ask him, he may already have it fixed in a pr.

[spycrab]

I checked myself on Catalina. Contrast looks good now.