Enable writing to Xmp.lr.hierarchicalSubject

[Alan Pater]

// Xmp.lr.hierarchicalSubject should be writeable but isn't due to this bug
// in libexiv2: http://dev.exiv2.org/issues/784

The above comment is from line 875 in PhotoMetadata.vala

The referenced bug in exiv2 is resolved and is targeted for exiv2 0.25.

The attached image had it's XMP properties written by my patched version of exiv2 using the following command:

     exiv2 -M"add Xmp.lr.hierarchicalSubject Level1|Level2" exiv2.784.jpg

Shotwell related bug is: https://bugzilla.gnome.org/show_bug.cgi?id=718923

I have tested with gexiv2 and it works there as well, after an initial glitch.

      https://bugzilla.gnome.org/show_bug.cgi?id=712477

[Alan Pater (alan-pater)]

Exiv2 0.25 has been released with support for writing Xmp.lr.hierarchicalSubject