added hide_mode option

With `hide_mode`, the dropdown hide method can be changed to prevent different problems. 

* `hide_mode: null` (default): uses the old `"left": -9000` to be backward compatible, but is problematic with very wide pages and with RTL.
* `hide_mode: "display"`: uses display hide/block, should be the most performant solution, but may result in uncorrect position/dimension calculations
* `hide_mode: "visibility"`: should be the most compatible version


Tested on a page with ~12000px x ~3000px and ~300 chosen dropdowns, with current Chrome & Firefox.