data-grid column sorting arrows bug & styling

hey @ansgar, any chance of getting the column sorting arrows to be more visible?

also there's a bug with the current thin arrows where they don't switch direction when sorting. see recent issue comment:

https://github.com/HeidiSQL/HeidiSQL/issues/618#issuecomment-647536555

(sorry for the ping spam, i'm sure you have more important bugs/features to work on)

thanks!