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>