Added helper functions for setting initial window size:

Added helper functions for setting initial window size:
* inline MakeDefaultNSRect makes an NSRect with position (10.0,10.0) and
  size based on the size passed to Create run through (Width|Height)Default
  This NSRect is to be used with the initWithFrame: initializer.
* SetInitialFrameRect is called after the window has been added to its parent
  and (if applicable) sized to fit.  If -1 is specified for a dimension then
  the fit/default size is kept.  If not, the window is sized to the specified
  size. It will be positioned in wxWindows coordinates (0,0==TL).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@22809 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775