The actual purpose is to help Plasma taskbar associate taskbar items
with .desktop files for the purpose of creating launchers. LibreOffice's
WM_CLASS handling is rather broken, but the class part of it usually
ends up being 'libreoffice-<component>', which however doesn't match
names of .desktop files or executables. I fear renaming .desktop files
would cause too much breakages in various places that expect the current
names, so adding StartupWMClass will help Plasma match them.

Change-Id: Ia946307b429d9a7c00408b2b508726fdc88908ac