Cocoa: Implement support for child windows.

Cocoa: Implement support for child windows.

On OS X, child windows (in the Qt sense) are not
windows. Add special case to window creation that
links the content views instead of creating a
NSWindow. Add a similar special case to
setGeometry().

Refactor window (re)creation into recreateWindow(),
which is called from both the QCocoaWindow
constructor and setParent().

m_nsWindow may now be null, add null-pointer checks
to all usages. Change winId() to return the
m_contentView pointer instead of m_nsWindow.

QGLWidget now works, but probably has sibling window
stacking issues which we won't be able to fix without
moving to client-side compositing.

Change-Id: I2e74cf27734dba7076c150e0d8341f0a62d3de2d
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>