This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

Publish the Client API, preparing to move more code into the Client
class.

Replaces GLib's GError.

playlist and player_control are deprecated.

Don't use the global variable "g_playlist".

Fixes regression.

Let the database plugin do the match.

Merge locate_song_search() and locate_song_match().

Fold the case during construction, without having to create another
copy.

Prepare to use the DatabasePlugin C++ API directly.