Multiple Optgroups

I'm new to this so please forgive me if this is just shit.
I also wanted to create an extra pull for each of these commits, but I can't seem to figure out how to do just that, sorry.

728478e7ac71e5dc5f7523fdc60125cc564b5d75 introduces a fix for when you are using multiple optgroups inside your selectbox
  * I've made a comment on the most important line on that commit with further details. In short: the code assumes that `j` resets after each optgroup (which it does not), so the counting is off and checkboxes are duplicated and assigned to the wrong optgroups

de8375d1524f27ba74e69985c5d99360787278e7 fixes css styling in firefox
  * apparently firefox does not like `background-position-x` and `background-position-y`

45b1ce58c0ba64599cd7c4ec0097f3e95522c2f4 fixes the issue of not being able to close the dropdown with a click on the div-container
  * for this I've removed the click event on .zselect and expanded the condition in the document.onMouseUp event