Add LinkTarget::createFragmentTarget() function

Add LinkTarget::createFragmentTarget() function

The createFragmentTarget function allows for switching the fragment on a
target in an easier way. TitleValue already had a now-renamed
createFragmentTitle function (no uses outside of tests), and an
implementation was added for Title.

This will also help with reducing the amount of public usage of
Title::setFragment(), which is deprecated.

Change-Id: I1e8ba2f85e748b1b4394fb2f2a1ccce69cf6e3c5