Currently it is possible to group window buttons. Here are the Preferences for the Window List panel applet:
Currently window buttons placed to the left represents windows that was opened first:
With "Always group windows" enabled
Feature request: Automatically sort the window buttons by name
Same principle as "Always group windows" but not collected in a group window list button (see above). So even if I opened the second Libreoffice Calc document after the Libreoffice Writer document (see section), both Libreoffice Calc documents are sorted next to each other:
