Bug 795693 - GIMP doesn't enable visibility of vector layers in PSD

In the file-psd plug-in, ignore the 'irrelevant' flag of layers
when loading PSD files; in particular, don't hide such layers
unconditionally.  The 'irrelevant' flag seems to indicate that the
layer's content can be entirely derived without using the layer's
pixel data, and not that the layer itself it irrelevant.