The about dialog box construction was awkward and brittle using
several depricated interfaces. This changes it to use a more generic
dialog creation and uses the response API to simplify click
processing.
It would be even better to use the gtk about dialog but that is a more
invasive change.
The netsurf.h header should *only* contain the registration, core
initialisation and finalisation methods. Version information is best
placed in its own header.
Also remove any unneeded inclusion of this header limiting it to
solely the places the relevant API is required.