add ability for sliders to be double-clicked to return to default value, similar to how native sliders work

Example:
The native aseprite hue/sat/bright effect lets you double click a slider to reset it to its default value. I don't think we can do this via the API at the moment. Would be very useful :)

Start by reviewing the slider API and the native Aseprite hue/sat/bright effect behavior referenced in the issue. Determine how a slider's default value is represented and where double-click handling belongs. Done means API sliders can be double-clicked to restore their default value, with behavior matching native sliders.

Written by the indexing model from the issue text.