Frequency of occurrence of lemmas in meanings

1. New class PWInit for creating and updating tables with prefix 'pw_'
2. New method PWInit::count_frequency_lemma_in_meaning() Counts
frequency of occurance of lemmas in meanings and writes to field
`pw_lemma_LANG_CODE.frequency`, if this lemma does not exist in table,
that it added there with origin=2 and meaning_id where it has be found.
3. New example frequency_lemma.php displays table with lemmas,
frequencies, origins, meanings
4. New method PWString::restoreCase transforms case of $word in the
correspondence with $mask_word.