writing: Replace zathura with sioyek

Replaced or extended all default references to zathura with
sioyek so it will automatically take on any tasks meant
for pdf reading. With the current configuration, hardly a
change should be noticed.

terminal/.config/vifm/vifmrc

@@ -358,7 +358,7 @@ set classify+=' ::*.doc,,*.docx::, ::*.xls,,*.xls[mx]::, ::*.pptx,,*.pp
 " Pdf
 filextype *.pdf
         \ { view as rich file }
-        \ zathura %c %i &, apvlv %c, xpdf %c,
+        \ sioyek %c %i, zathura %c %i &, apvlv %c, xpdf %c,
         \ { edit text content }
         \ pdftotext -nopgbrk %c - | nvim
 fileviewer *.pdf pdftotext -nopgbrk %c -
@@ -366,14 +366,14 @@ fileviewer *.pdf pdftotext -nopgbrk %c -
 " PostScript
 filextype *.ps,*.eps,*.ps.gz
         \ {View in zathura}
-        \ zathura %f,
+        \ sioyek %f, zathura %f,
         \ {View in gv}
         \ gv %c %i &,
 
 " Djvu
 filextype *.djvu,*.epub
         \ {View in zathura}
-        \ zathura %f,
+        \ sioyek %f, zathura %f,
         \ {View in apvlv}
         \ apvlv %f,