indentifiers in IDF - the SCM function isValidRevision has been replaced
by a validateRevision() method which returns one of three states,
valid, invalid or ambiguous.
The source view can then act accordingly and display disambiguate view
for the latter, so the user can select for which revision he actually
wants to execute the requested action. Also, invalid revisions now lead
to another separate view, telling the user that it is invalid / does
not exist and pointing him optionally to the help page where he can read
further how to access his repository to push the first changes into.
(partially resolves issue 525)
