CMakeLists.txt: Disable building of audio unit
Internal clean up of layout interface

* gui_config.hpp: Rename Orientation to Orientations.

* UILayoutItem: Introduce required directions interface method.

* Use more sane defaults for max/min & size hints where appropriate.
types: Introduce MAX_INT static const var
Transformable: Introduce set_bounds method variants & constructor
Rect: Introduce position & size helper methods
Refactor Decorator API

My first attempt at consolidating the interfaces, removing redundant
pieces and providing a more friendly interface for extending from.

* Decorator is now an abstract base class; MinimalDecorator &
FinalFantasyDecorator now inherit from.

* Finish dirty update logic in MinimalDecorator & FinalFantasyDecorator.

* UIWidget::validate_dimensions method is no longer needed and might be
removed in the future.
Initial working base implementation of VBoxLayout

(See build/tests/VBoxLayoutTest)

* Implement UILayout::update logic.
* UIWidget: Layout initialization logic implemented in ::set_layout.

* VBoxLayoutTest: Remove LayoutAPIUsingSmallBitmapFont unit test.
* VBoxLayoutTest: Add additional item strings for testing layout control.