Fix library search

Library search was broken during the song information refactoring and should be fixed by the following 2 commits.

Commit 2bee fixes a typo (wrong nesting of parentheses) introduced in the rework which yields the following error:

```
Traceback (most recent call last):
  File "/mnt/files/build/sonata/sonata/library.py", line 1289, in libsearchfilter_do_search
    if not regexp.match(row.get(searchby, '')).lower():
AttributeError: 'NoneType' object has no attribute 'lower'
```

Commit 5b42 provides a missing method to the MPDSong class to prevent the following error:

```
Traceback (most recent call last):
  File "/mnt/files/build/sonata/sonata/library.py", line 1296, in libsearchfilter_do_search
    allstr = " ".join(row.values())
TypeError: 'NoneType' object is not callable
```