A previous Vocalinux install can leave ~/.local/share/applications/vocalinux.desktop
and ~/.local/bin/vocalinux(-gui) behind. When their target or venv is gone they
start nothing, yet they sit next to Voculinux in launchers.
The one-time legacy import now also checks those three paths, after the tray is
up, with stat calls and small reads only. A launcher is stale when its target is
missing, a link is dangling, or a script names a missing interpreter or venv
entry. A working install is never reported. One notice names the stale files and
a marker in the new data directory keeps it to once. No file is removed or
modified.
Fixes#52
xavierk