Replace UiStack with something better

[droc101]

The current UiStack system is kind of a mess and should be replaced with something that is more capable.

Feature requests for new system:

• Multi-Window / Popups
• Scrolling
• Layout asset & Layout editor in GAME SDK
• Controls
  • Label
    • Font size & color
  • Image
    • Size mode (stretch, fit, fill, etc.)
  • Tabs
  • Dropdown/Combo Box
  • Text Box
  • Number Box
  • Scroll area
  • Link button (both to web and to callback function)
  • Button
    • "Primary" button, focused by default
    • "Cancel" button, pressed when esc is hit
  • Check Box
  • Radio Button
  • Float slider
  • Int slider
  • Input Action Box
  • Window
    • Modal/non modal
    • Resizable/not resizable
    • Title
    • Icon (or none)
• Specify focus order for tab, shift+tab, and dpad directions independently
• All controls in a "layout" are given unique names so they can be accessed from engine code
• Simultaneous and/or auto switching controller & keyboard support