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/debugger/debuggerplugin.cpp

@@ -1120,6 +1120,7 @@ bool DebuggerPluginPrivate::initialize(const QStringList &arguments,
     m_visibleStartAction.setAttribute(ProxyAction::UpdateIcon);
     m_visibleStartAction.setAction(&m_startAction);
 
+    m_visibleStartAction.setObjectName("Debug"); // used for UI introduction
     ModeManager::addAction(&m_visibleStartAction, Constants::P_ACTION_DEBUG);
 
     m_undisturbableAction.setIcon(interruptIcon(false));