Refactor button hotspot manangement.
Make ButtonWidget::_hotspots private.
Add layout classes.
Use LayoutBounds for view bounds.
Use LayoutBounds in ImageWidget and StringWidget.
Use LayoutBounds in ButtonWidget.
Add method ButtonWidget::UpdateLayout.
Fix bug in TextureAtlas::AddTextureImage.
Make method TextureAtlas::NewTextureImage public.
Add GetGraphicsContext accessor method to Widget and View.
Rename method center -> mid in bounds1f and bounds2f.
Replace Bounds with SetPosition and GetPosition in StringWidget.