Add manifest for okular and fix open with for PDF

* src/inst-section.nsi: Change shortcut name.
* patches/okular/0001-Add-GnuPG-specific-manifest-to-okular.patch: New.
* src/uninst-okular.nsi: Fix new values.
* src/inst-okular.nsi: Update registration.

The filetype registration differs slightly from Kleopatra because
this is now fully according to the book. The open with menu
never seemed to work. But it turned out that the missing versioninfo.rc
was the reason for that and not the registry values.