Allow compiling test programs with only selected plugins.

Caused by commit 0b4fa41a

Make attributes const, don't allow partial initialization.
Instantiate the CdioParanoiaInputStream object at the end of
input_cdio_open().

Turns out ucol_getSortKey() does not what I thought it does.

Fixes the broken "search" command.

The attribute must be 0..100 and not 0..1024.  Previously, the code
was inconsistent.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

This case is handled already by playlist_open_in_playlist_dir() (via
playlist_mapper_open()).  And the call didn't work anyway.

Eliminates some overhead and some duplicate code, and fixes a serious
bug: the old code did not append the ".m3u" suffix, and thus the
"load" command was completely broken for stored playlists.  D'oh!

Fixes missing song length on high-latency Opus files.

According to tests with 320 kbit/s opus files with 60ms packets, we
need to skip up to 29 kB.

Turns out the lock-free code using atomics was not thread-safe.  The
given callback could be invoked by GLib before the source_id attribute
was assigned.  This commit changes the DeferredMonitor class to use a
Mutex to block the event loop until source_id is assigned.  This bug
does not exist in the 0.19 branch because it does not use the GLib
main loop anymore.