ScriptFu: bug: new-style dialog lacks slider widget

A script arg declared SF-ADJUSTMENT...SF-SLIDER should have a widget that is a slider. It does in the old-style dialog, but not in the new-style dialog.

For a property of type DOUBLE, GimpProcedureDialog defaults the widget to a entry with spinner. Thus ScriptFu must create a "custom" widget that is a slider with spinner.

Discovered in related #12157 (closed).