Add general UI introduction

Adds a Help > UI Introduction, which highlights basic aspects of the Qt
Creator UI.

The pages to show are defined in a format that can later move into a
text based configuration file like JSON. It specifies an optional object
name of a widget to highlight, a title, a brief description and a longer
description (potentially with tables, lists and other simple HTML).

Fixes: QTCREATORBUG-21585
Change-Id: Idb64c87e1d752bc24437588278093a96be0eeddb
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/coreplugin/fancyactionbar.cpp

@@ -373,6 +373,8 @@ void FancyActionBar::addProjectSelector(QAction *action)
 void FancyActionBar::insertAction(int index, QAction *action)
 {
     auto *button = new FancyToolButton(action, this);
+    if (!action->objectName().isEmpty())
+        button->setObjectName(action->objectName() + ".Button"); // used for UI introduction
     button->setIconsOnly(m_iconsOnly);
     m_actionsLayout->insertWidget(index, button);
 }