OSX color dialog can be accepted repeatedly

QNSColorPanelDelegate::finishOffWithCode sets mResultSet = true
and it needs to be set false again the next time the dialog is shown.

Task-number: QTBUG-31566
Change-Id: If1d4bb9d4e76273c6423f5bf2ae37790e8a9704e
Reviewed-by: Liang Qi <liang.qi@digia.com>

src/plugins/platforms/cocoa/qcocoacolordialoghelper.mm

@@ -274,6 +274,7 @@ static NSButton *macCreateButton(const char *text, NSView *superview)
 - (void)showModelessPanel
 {
     mDialogIsExecuting = false;
+    mResultSet = false;
     [mColorPanel makeKeyAndOrderFront:mColorPanel];
 }