Delay widget creation of options pages till their category is shown

Also add the alternative way: IOptionsPageProvider, which states
a category, and is asked for a list of options pages for that
category when it is shown.

Task-number: QTCREATORBUG-3131

src/plugins/qmljseditor/quicktoolbarsettingspage.cpp

@@ -161,6 +161,8 @@ QWidget *QuickToolBarSettingsPage::createPage(QWidget *parent)
 
 void QuickToolBarSettingsPage::apply()
 {
+    if (!m_widget) // page was never shown
+        return;
     m_widget->settings().set();
 }