Modify FileValidator extension checking to be more generic

[peterfpeterson]

Rather than looking for the extension to be the stuff after the final ".", have it look for the file ending. This way things that have double extensions (i.e. .tar.gz, .hdf.nxs) as well as extensions that don't start with "." (i.e. _neutron_event.dat, _pulseid.dat) can be matched against. The functionality is similar to python's endswith is the correct analogy. For filters that still want to have an extension start with a ".", they can just add it to the specification of the filter. Most users won't notice that the "." is no longer required.

[peterfpeterson]

This issue was originally trac ticket 1302