MacOSX: Make wxWebviewWebkit capture hotkeys less eagerly.

On MacOSX, the wxWebviewWebkit will eagerly capture the hotkeys despite the fact
that it might be the one in focus. This will interfere other widgets' functions.
It should only handle the hotkeys when it is the firstResponder.

More about firstResponder:
https://developer.apple.com/documentation/appkit/nswindow/1419440-firstresponder

src/osx/webview_webkit.mm

@@ -747,6 +747,10 @@ -(id)validRequestorForSendType:(NSString*)sendType returnType:(NSString*)returnT
 
 - (BOOL)performKeyEquivalent:(NSEvent *)event
 {
+    if (self.window.firstResponder != self) 
+    {
+        return NO;
+    }
     if ([event modifierFlags] & NSCommandKeyMask)
     {
         switch ([event.characters characterAtIndex:0])