Resolves #1192

`/lib/plugins/<plugin>/deleted.files`
which list files that can be removed e.g.
```
# This is a list of files that were present in previous plugin releases
# but were removed later. An up to date plugin should not have any of
# the files installed
action.php
folder/file.txt
```

Выравнивание и разбиение по образцу en/lang.php, унификация "директорий" и "папок", "расширений" и "дополнений"