Combobox with 3 or more page groups

[PLUS-RobertEhrenleitner]

Dear Kyle,

I did my best to apply the modifications we at the PLUS do to the plugin. It took me some more effort because we made it rather hacky and ugly until now, but now I wanted to contribute a clean code.

First things first: We introduced a new term, "page group" (more precisely, we introduced it in German, "Seitengruppe"). A page group is logically a toplevel page with all its subpages. Technically, a page group is identified by its toplevel page ID alone.

This modification prevents Nested Pages from loading and listing all the page groups if they exceed a certain number. If so, a page group needs to be selected from a combobox first in order to list it. The combobox is always there, so that the view can always be restricted.

While our modification usually includes a fixed limit of 3 page groups for the selection to be required, the provided modification lefts it configurable (but defaults to 3).

Please tell me if there is something I have to fix or I can improve.

I could not get Gulp running until now, because npm install fails for me. So, if this contains some tests or code beautifiers, I could not run them.

Cheers,
Robert.