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/progressmanager/progressmanager.cpp

@@ -300,6 +300,7 @@ void ProgressManagerPrivate::init()
     readSettings();
 
     m_statusBarWidget = new QWidget;
+    m_statusBarWidget->setObjectName("ProgressInfo"); // used for UI introduction
     auto layout = new QHBoxLayout(m_statusBarWidget);
     layout->setContentsMargins(0, 0, 0, 0);
     layout->setSpacing(0);