installer: cleaner support for splitindex and pdfcrop

installer: cleaner support for splitindex and pdfcrop

- it turned out that the hack to install a stripped-down Perl directly to MiKTEX is not only error-prone, requires admin permission but that it is also not safe because MiKTeX changes the folder structure from time to time. With the new solution Perl is installed directly for LyX like we do for ImageMagick, Python etc.
- for pdfcrop support there is a bug in pdfcriop that the requirement is hardcoded to Ghostscript 32bit - I contacted the pdfcrop author to fix this