A special case is now supported when no VCS binary is specified (left
blank in options) then a default (fallback) binary is used instead.
Class VCSBaseClientSettings is now equipped with defaultBinary() and
setDefaultBinary(). Descendant classes can define the default binary
to be used when needed (see BazaarClientSettings for example).

Task-number: QTCREATORBUG-5666

Change-Id: I74c79df03f820996b31a1cd05201bd88ba5dd081
Merge-request: 364
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt.nokia.com/2996