Improve FlxUIDropDownMenu for large data lists

[Gama11]

With a lot of data, the height of the drop down menu will simply exceed the screen height. There are two options to solve this issue in my opinion:

• Add the option to have multiple columns, using space horizontally
• Add a scrolling behaviour - only part of the data list is shown, and when the mouse cursor hovers over the end of the list at the bottom of the screen, the list scrolls down

Ideally, we would have both of those options.

[larsiusprime]

Great ideas!

[DalekCraft2]

This is a very late addition to the thread, but could the scrolling behavior be implemented as well? It could also work with the scroll wheel.