idalib: search for desktop entry in file on Desktop

[williballenthin]

not found in ~/.local/share/applications but on the desktop directly

[williballenthin]

today we don't search for desktop entries because we can use the config file, but this is only written if idalib has been manually installed at least once. so we can use this strategy to handle the case where people have ida installed but haven't mucked with idalib yet.