Re-enable visualIDFromVisual
Add bindings for VisualInfo functions

This commit adds a VisualInfo type, a VisualInfoMask type and
enumeration constants, and bindings for XGetVisualInfo() and
XMatchVisualInfo().
Add an emptyVisualInfo constant

This simplifies the creation of new VisualInfo instances in user code.
Export VisualInfo(..) from Xlib

The VisualInfo type and accessor functions can now be used without
importing Xlib.Types directly.
Merge pull request #18 from anthonyde/visualinfo

Add bindings for VisualInfo functions