Making this validation code public allows projects to validate a
GParamSpec name before creating it. While hard-coded GParamSpec don't
need this, we can't afford crashing the main program for dynamically
generated GParamSpec from user-created data.
In such case, we will need to validate the param names and return errors
instead of trying to create a GParamSpec with invalid names.
Includes modifications from Philip Withnall and Emmanuele Bassi to
rearrange the new function addition and split it into one function for
GParamSpecs and one for GSignals.