libdeng2|ByteRefArray: Added a default constructor, data accessors
libgui: Cleanup, apidoc improvements
libgui: Added de::Image, a wrapper that extends QImage

The pixel formats supported by QImage are only partially compatible
with OpenGL (ES). de::Image defines a full set of GLES friendly
formats that can be used to pass raw pixel data (or QImages) to
GLTexture.
libgui|GLTexture: 2D textures, cube maps, automatic/manual mipmaps

Implemented GLTexture with management of 2D textures and cube maps.
Image content can be uploaded to any mipmap level. Mipmaps can be
generated automatically, by request, or set up entirely manually.