[poppler] Poppler 0.51.0 released

[Albert Astals Cid]

Available from
http://poppler.freedesktop.org/poppler-0.51.0.tar.xz

Changes against 0.50.0 include:
core:
 * Check for error from NSS in SignatureHandler construct. Bug #99363
 * Add Form[Field|Widget]::setPartialName
 * Fix memory leak in PDFDoc::markAnnotations

qt5:
 * Implement digital signature support. Bug #94378
 * Add Poppler::FormField::setName
 * Fix segfault/assert if LinkDestination is constructed with invalid input 
string. Bug #99357

utils:
 * pdfunite: add fields to AcroForm dict. Bug #99141

Testing, patches and bug reports welcome.

Cheers,
  Albert