libgui: API doc improvements
libdeng2|Id: Added isNone() method and a qHash() function
libdeng2|Rectangle: Added expanded(), fromSize() constructor, union
libdeng2|Vector: Added Vector4ub typedef, suitable for RGBA colors
libgui|DisplayMode: Note about why it uses a C API
libgui|GLTexture: Notify specialized textures about texture binding

When a texture is taken into use, a specialized texture may need to
take some preparatory steps.
libgui|Image: Added basic image editing methods (using QImage)

Clearing the image, filling rects, copying parts of the image.
libgui: Implemented image/texture atlases

Added Atlas, RowAtlasAllocator and AtlasTexture.

de::Atlas is an abstract class and provides the primary interface for
image-based atlases. It optionally maintains a backing store.

de::Atlas::IAllocator is the interface for an atlas allocator that
decides where content is placed inside the atlas.

de::RowAtlasAllocator is a row-based allocator for content whose
heights are fairly constant (e.g., text strings).

de::AtlasTexture is a concrete atlas class that uses a GLTexture as
the physical storage medium.