Add API to hide expanded folds except on hover

[bobbylight]

Describe the solution you'd like
Modern code editors often hide "expanded" fold icons unless the mouse is hovering over the gutter. In particular, editors whose fold icons resemble those recently added to RSTA as the "modern" style in #458. We should add an API to Gutter to support this, and do so out-of-the-box for the "modern" fold style.

Are there any workarounds?
No

Additional context
None