Clean up layer list styling on MacOS

[ajashton]

• the expand/collapse button is ugly - can it be a frameless > like on Linux?
• need some space between the color swatch and the checkbox
• the white scroll area background looks awkward on the grey QTabWidget background/frame (especially when the scroll bar is visible) - not sure what we can do about this though

Maybe it would make sense to use an entirely different Qt theme by default, like QGIS does.