Add property for disabling lum-sat square rotation

[callaa]

I received this feature request for my program that uses Qt-Color-Picker. Picking shades with the non-rotating color wheel is easier, and this style is fairly commonly used, including by programs such as Krita, Photoshop and Sai.

[mbasaglia]

OK, after merging I'll make it have the black part on the bottom as it would look nicer that way.

[callaa]

Thanks!

[mbasaglia]

I've got an idea: maybe I'll replace rotatingSquare with a set of flags and add more style options. For one thing I'd like to make it a triangle, maybe I can also add support to display it in the HSL color space (right now it's HSV).