NS{Application,Window}: define modal constants

Define constants and stubs related to modal sheets in GUI for increased
compatibility.

Headers/AppKit/NSApplication.h

@@ -750,6 +750,16 @@ NSShowSystemInfoPanel(NSDictionary *options);
  */
 APPKIT_EXPORT NSApplication	*NSApp;
 
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_3, GS_API_LATEST)
+enum
+{
+  NSModalResponseStop = -1000,
+  NSModalResponseAbort = -1001,
+  NSModalResponseContinue = -1002
+};
+typedef NSInteger NSModalResponse;
+#endif
+
 #if defined(__cplusplus)
 }
 #endif

Headers/AppKit/NSWindow.h

@@ -87,6 +87,13 @@ enum {
   NSScreenSaverWindowLevel = 1000  // 12
 };
 
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_9, GS_API_LATEST)
+enum {
+  NSModalResponseOK = 1,
+  NSModalResponseCancel = 0
+};
+#endif
+
 enum {
   NSBorderlessWindowMask = 0,
   NSTitledWindowMask = 1,
@@ -810,6 +817,14 @@ PACKAGE_SCOPE
 - (void) setDisplaysWhenScreenProfileChanges: (BOOL)flag;
 #endif
 
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_9, GS_API_LATEST)
+#if GS_HAS_DECLARED_PROPERTIES
+@property (readonly) NSWindow *sheetParent;
+#else
+- (NSWindow *) sheetParent;
+#endif
+#endif
+
 @end
 
 @class NSToolbar;

Source/NSWindow.m

@@ -5653,6 +5653,11 @@ current key view.<br />
   return _attachedSheet;
 }
 
+- (NSWindow *) sheetParent
+{
+  return nil;
+}
+
 @end
 
 @implementation NSWindow (Toolbar)