cocoa: Avoid warning related to multiple handleEvent: definitions

Avoid compiler confusion as to which method signature to use for the
handleEvent: selector on OSX >= 10.6 by making the variable type-safe
as opposed to generic 'id' type.
Requires moving the variable definition to after the class definition.

----8<----
ui/cocoa.m: In function ‘cocoa_refresh’:
ui/cocoa.m:997: warning: multiple methods named ‘-handleEvent:’ found
/System/Library/Frameworks/AppKit.framework/Headers/NSTextInputContext.h:84: warning: using ‘-(BOOL)handleEvent:(NSEvent *)theEvent’
ui/cocoa.m:272: warning: also found ‘-(void)handleEvent:(NSEvent *)event’
----8<---

Reported-by: Alexandre Raymond <cerbere@gmail.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Tested-by: Alexandre Raymond <cerbere@gmail.com>

ui/cocoa.m

@@ -63,7 +63,6 @@ typedef struct {
 } QEMUScreen;
 
 NSWindow *normalWindow;
-id cocoaView;
 static DisplayChangeListener *dcl;
 
 int gArgc;
@@ -278,6 +277,8 @@ static int cocoa_keycode_to_qemu(int keycode)
 - (QEMUScreen) gscreen;
 @end
 
+QemuCocoaView *cocoaView;
+
 @implementation QemuCocoaView
 - (id)initWithFrame:(NSRect)frameRect
 {