alias generator desktop files

Merged Harald Sitter requested to merge work/aliasfor into master

this allows UIs to know that this is really okular and may show okular instead (considering the generators are nodisplay=true)

specifically this allows the xdg-desktop-portal-kde to show okular in its appchooser dialog even though the actual mimetype match happend against (e.g.) okularApplication_pdf.desktop and is NoDisplay

Edited by Harald Sitter

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