filter .nx file when looking for a bliss scan

Today the 'nxtomomill h52nx' widget takes as input any .h5 file or nexus file. Even if bliss files are nexus file it can be very confusing to the user. So we should only allow .h5 / .hdf5 in this widget.

changed milestone to %1.4

added enhancement label

assigned to @payno

mentioned in issue #1284

mentioned in commit e4001d56

mentioned in commit f036f2b3

closed with commit e4001d56