Fix list height of DropDownPopUpContentManager

Hello Josh,

I noticed an issue using PickerList & DropDownPopUpManager when the list is taller than the available space, it's height is not limited to the available space automatically.
I checked the others IPopUpContentManager and they dont have this issue so I fixed it in the DropDownPopUpContentManager.

I hope you'll accept this patch :)
[Fix] DropDownPopUpContentManager now set maxHeight instead of height
Merge pull request #858 from Fraggle/patch-1

Fix list height of DropDownPopUpContentManager