Instead of using @borders for drag handles, we use @theme_fg_color at 40% opacity. Because these are widgets intended to be interacted with by users instead of drawing borders, we should use a semantically appropriate color which updates according to whether a user has a dark theme selected or not. This ensures that the drag handles contrast with the background no matter what stylesheet the user is using.
This improves the contrast of these widgets in the light theme from 1.7:1 to 2.9:1, which is still rather low, but greatly improves the usability of these widgets without having too large an impact on the look of the widgets in-situ. The contrast ratio is unchanged in the HighContrast style (4.0:1).
This also improves the situation if we eventually adopt a dark style preference for users, and improves the portability of the application.
There's a dedicated icon for drag handles - we should make use of
it. Also, change the color of the handle so it looks like it is
part of the row.
Fixes#590
