Improvements to HPGL import

Currently, the HPGL import does not have the option to import the file using page center as origin. This option is available when exporting an HPGL file from Inkscape and should also be available in the import of files to allow the correct preview of the files. Also, the current import uses a hard coded page size. This leads to incorrect orientation when importing HPGL files that were exported in a different format as HPGL code does not contain page size info. I have created a new HPGL import menu with page size selection and made adjustments to the HPGL decoder to address these problems.