Add simple front-ends for Poppler utils

[Adam Reichold]

Often users want to do simple operations like merging documents or extracting pages which are supported by the Poppler utils they often have already installed but not available to them since they are command-line-based. Hence it would be rather useful if qpdfview could provide some limited front-ends for these CLI utilities, for example to control pdfunite (merging several files into one) and pdfseparate (extracting some pages from a file).