Added more variations on BasicMath::RoundToInt

Including RoundToUInt
Added a TODO to GUIElement
Added "SetText" to GUITextBox
Fixed GUIFormattedPanel positioning for good
Removed realtive positioning in panel stuff outside Render()
Fixed small bug with mouse pos calc in GUIManager
Improved text box positioning (must specify height)
Fixed default values for some GUIElement fields
Fixed GUISlider to use GetColor() and SetColor()
Added GUICheckbox

project files also contain some files that haven't been committed yet,
unfortunately
Updated GUITestWorld with new fixes/GUIElements