This fixes thumbnailing for XDG desktops. The new script doesn't depend on GNOME libs and works either with python3 or python2.
Also, a thumbnailer config file is added to the installation in CMakeList.txt
Refer to `https://reproducible-builds.org/docs/source-date-epoch/`:
"SOURCE_DATE_EPOCH is a standardised environment variable that
distributions can set centrally and have build tools consume this in
order to produce reproducible output."
