Fix #1445: Use xcrun to find CpMac

This should make the DMG building step more reliable on all current and future
Xcodes, by using xcrun to find the path to CpMac.

Fix #1667: Docs about SDL_CreateWindow and flags.

Updates the docs to say you can use SDL_WINDOW_HIDDEN, instead of (the ignored)
SDL_WINDOW_SHOWN.

Re-enabled other messagebox tests, moved the SDL_Init() call as late as possible to show the circumstances where it's needed.

Fixed bug 1970 - Cocoa message boxes ignore parent window requests

Ryan C. Gordon

Cocoa_ShowMessageBox() ignores the "window" field of SDL_MessageBoxData, which means you can't assign a parent window to a message box. This is particularly egregious on Mac OS X, because it'll actually make the NSAlert visually part of the parent window instead of just concerning itself with window focus.

Added testing of messagebox with a parent window

Fixed code example for SDL_GetWindowWMInfo()

Removed obsolete assertion code

Make sure a window is valid for a subsystem before using it in a messagebox

If the video system has been initialized, only use that message box system.
Don't pass a window from one video driver to the messagebox function of another video driver. This makes bad things happen. :)

Fixed compiler warnings in test program by using wrapped functions.