3.19.91
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 Fix syntax error in icons Makefile by Miguel Vaello Martínez.
 Prevent stylesheet file deletion after 'make clean' by Miguel Vaello Martínez.
 Fix duplicate buttons in headerbar of about dialog by Jishnu Mohan.
 Icon: provide a symbolic variant by Jakub Steiner.
 Translation updates.
 Format date created.
 Create lookup table for files so retrieval for renaming is n(1) operation.