Issue #1020: Use theme foreground color for range handle outline

This patch is an attempt to resolve issue #1020 (closed).

In the Levels tool (and other dialogues which auto-generate a range widget), the range handles use hardcoded constants to define the handle outline colors. RGB 0.6, 0.6, 0.6 for left handle, and 0, 0, 0 for the other two. While this is an improvement from using 0, 0, 0 on all three in dark mode, there was a further request to make them match the current GUI theme.

In discussions with @rangelovd, it seems preferable to leave the handle fills as they are (since black, grey, and white indicate their function). However, the Gradient Editor range handles already use the theme's foreground color for their outline, so to be consistent, we should probably use it here as well.

(@rangelovd Hi! Let me know if I've misunderstood anything from our discussions. Also, I can lower the outline's alpha value if you like - currently it's at the default 1.0, full opacity).