PCTLIBRARY-935: Adds mechanism to add new view handlers to the view fixture.

ViewFixture.addViewHandler does not allow existing handlers to be
overridden (an INVALID_PARAMETERS error is thrown if this is attempted).
(@ewant)
Merge pull request #476 from janhancic/develop-caplin-patches-viewfixture

PCTLIBRARY-935: Adds mechanism to add new view handlers to the view fixture