Disable hover/prelight effect on workspace buttons by default

There's some default GTK CSS for the hover effect which I personally find unappealing, and I think many would concur. I've mitigated it by adding this rule:

```css
#workspaces button:hover {
  box-shadow: inherit;
  text-shadow: inherit;
}
```

Maybe this could be done out of the box? Alternatively, prelight could be disabled altogether, but I think some may want to have hover effects.