Determine use-case(s) for QSignalDumper

QSignalDumper was already present when qtestlib was imported into the main Qt codebase in October 2005, and has hardly changed since then. The code was imported but the prior commit history was not, and the two developers who worked on the early version of qtestlib have not worked on Qt since 2013, so the original use-case(s) seem to have been lost to the mists of time.

Rather than simply documenting QSignalDumper as-is and doing the minimum necessary to keep it compiling, we should determine whether it is still of any value for current Qt users and then treat it accordingly (i.e. deprecate it, replace it, or enhance it).