Adjust fold-widget visibility based on folding-enabled

Slight adjustment to the visibility of `.ace_fold-widget` so that when the `.ace_folding-enabled` flag is toggled, so is the widget's visibility. This addresses the issue where the left-most edge of the folding widgets may still be visible after disabling code. folding
Merge pull request #1387 from rgriffith/folder-widget-css-improvement

Adjust fold-widget visibility based on folding-enabled