QNSView: Disable focus ring

Not only we don't need Cocoa to display it for us, but it also
seems to take a lot of time after the call to -[NSView
setNeedsDisplayInRect:].

While resizing a large and busy widgets window, we can decrease
the relative time spent in -[QNSView flushBackingStore:region:
offset:] from over 15% down to around 3%.

Change-Id: I2a8d51be28b77c4c3da2588d9a14e461e7910167
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/plugins/platforms/cocoa/qnsview.mm

@@ -164,6 +164,7 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
         }
 
         m_isMenuView = false;
+        self.focusRingType = NSFocusRingTypeNone;
     }
     return self;
 }