UIWidget: Make class non-copyable

* This is a good idea due to many reasons, i.e.: our widget's usage of
uniquely identifying numbers -- collisions could be really bad (think:
events, debugging and so on).

* IDrawable: Remove required implementation of IDrawable::clone; this
was necessary in order to make UIWidget & deriving classes non-copyable.
Nothing to see here, move along...

* examples/gui/listbox_ex1: Append third item.
* examples/gui/listbox_ex2: Use a truetype font for display.

* SDLApp: Add documentation notes.
UIWidget: Use relative positioning coordinates for layouts

* Same coordinate system as widgets.
VBoxLayout: Fix a few calc errs relating to spacing

* See previous commit file examples/gui.cpp for changes that were
intended to go with this.
Fix MSVCPP compile errs

* Include missing header file for std::unique_ptr, std::shared_ptr.
* Could not initialize static array in constructor.