vcsbase: add mechanism to ease management of VCS settings

Management of VCS settings was rather laborious (see BazaarSettings,
GitSettings, ...).
To overcome this, the base class VCSBaseClientSettings is now
equipped with a mechanism factorizing redundant code for VCS settings,
like operator=(), operator==(), writeSettings(), ...
Heir classes of VCSBaseClientSettings only need to declare the
settings (key + default value).

Bazaar and Mercurial plugins are impacted.

Change-Id: I7e4093a6d9476f3c8954f4d104f1b26ac185beb9
Merge-request: 2204
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt-project.org/4889
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>

src/plugins/mercurial/mercurialplugin.h

@@ -91,8 +91,6 @@ public:
     static MercurialPlugin *instance() { return m_instance; }
     MercurialClient *client() const { return m_client; }
 
-    QStringList standardArguments() const;
-
     const MercurialSettings &settings() const;
     void setSettings(const MercurialSettings &settings);