Simplified MTexture classes' Bind() behavior

Other rendering classes that don't use MTexture stuff, like Material,
mess up the static tracking of the currently-bound texture.
Fixed TextRenderer bug with each slot's string
Added static tracking of currently-bound RenderTarget
Added "CopyTo()" to KeyboardTextInput

Didn't want to risk allowing implicit copies of the class because of its
size.
Minor error-handling for FreeTypeHandler::GetChar()
Added color param getter/setter in GUIElement
Added default constructor for GUITexture/GUISlider
Fixed GUILabel bounds calculation
Fixed GUIFormattedPanel positioning
Created GUITextBox using KeyboardTextInput
Updated GUITestWorld for new GUI stuff like TextBox