Merge branch 'xdg-open-as-default-pdf-viewer' into 'master'

use xdg-open as the pdf viewer

See merge request olale55/liuthesis!31

Makefile

 # Use Bash shell expansion
 SHELL=/bin/bash
 #The program that you use for viewing pdf files
-PDFVIEWER = evince
+PDFVIEWER = xdg-open
 # Change name of this if you change the name of demo_student_thesis.tex, should be name of the main tex file
 TEXMAINFILE = demo_student_thesis.tex
 # Name that you would like for your resulting pdf file, without extension.