Make CompWindow::constrainNewWindowSize testable

Move CompWindow::constrainNewWindowSize out into its own namespace under compiz::window::constrainment and call it something a little more descriptive like constrainToHints with provided sizeHints returning taking:

CompSize
compiz::window::constrainment::constrainToHints (const XSizeHints *hints,
                                                                                             const CompSize &size,
                                                                                             unsigned int ignoreHints);