Currently the file "src/XDGData/FreeCAD.thumbnailer" gets installed, but
the associated freecad-thumbnailer script isn't. Install the script,
too.
While here, have the paths to the default icon and freecad-thumbnailer
script follow ${CMAKE_INSTALL_DATAROOTDIR} and ${CMAKE_INSTALL_BINDIR}
respectively.
The QGuiApplication::setDesktopFileName() call was changed in [1] but
the desktop file and other files that reference it were not.
As shown by Looking Glass [2] on X11 updating these files is still
not enough for GNOME to associate the process with the desktop file,
a StartupWMClass entry is added to the desktop file to fix this.
[1]
129d5882a7 ("Migrate domain name from freecadweb to freecad (#9352)", 2023-04-24)
[2]
https://blogs.gnome.org/mclasen/2016/11/15/help-my-app-icon-is-missing/
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."
