pep file stores relative path naively

[Mortal]

Steps to reproduce:

1. Switch to directory foo/bar and create a new pdfannotater project on file ./baz.pdf. Close pdfannotater.
2. Switch to directory foo and open the pep-file ./bar/baz.pdf. Save or export the project.
   Expected outcome:
   After step 2, the project is saved as foo/bar/baz.pep.
   Actual outcome:
   After step 2, the project is saved as foo/baz.pep since Project.path is a relative path. Unfortunately, this can easily lead to data loss if foo/bar/baz.pep is reopened (an old version) and resaved on top of foo/baz.pep (overwriting a newer version).