Editor: make Global Variables pane list to have a tree folder structure

[ivan-mogilko]

A request from a user on forums:

So for a while I've always wondered if there could possibly a small improvement to the global variables usage, mainly would it be possible to add some folder options so we can store variables in a slightly better user friendly structure.

Right now the global vars are displayed in a simple list view (afaik), having folders would require to:

1. In Game.agf: support serializing global variables in subfolders, similar to many existing game entities;
2. On UI: replace list box with something supporting folders and columns at the same time, get an existing 3rd party component, or code your own.

[jumpjack]

Current implementation is "mixed": there is one single item in the "listboxes panel" on the right, but no listboxes associated to it: double clicking on the "Global variables" item opens up a window which lists variables:

It remembers me how Windows register (which supports folders) is organized: