Set up the UI framework, load UI data from .xml

- Set up the UI framework
- Use pugixml library to load UI data from the .xml files
Set up HandleInput functions, add Focus state

- Set up HandleInput functions call chains that pass input from the
active screen to its children
- Let the Menu handle the Up/Down keys
- Let the Button handle Enter key and call function pointer OnClick
- In MainScreen, set up the OnClick functions for each buttons in the
menu
- Add focus state to UIComponent, change the text color when an item is
focused