This alleviates a problem in which 'volume +1' cannot be undo by
'volume -1' when using alsa hw mixer.

Closes #104

Fix breakage from commit bea5681f

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Yet another potential crash bug fix.

Fixes potential crash bug.

Doesn't make a practical difference - but it's more correct this way.

Changed AlsaMixerPlugin to use the get and set normalized functions from volume_mapping of alsa-utils/alsamixer
Changed volume_mapping set volume to be for all channels and not per channel
added volume_mapping files to Makefile.am

The old struct config_param remains only for top-level string options.

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Move code from AlsaMixerPlugin.

EventLoop::AddCall() and EventLoop::AddIdle() are unsafe, because we
can't cancel those calls.

This deadlock was a regression by commit 8e38b4f8.  Since we currently
can't resolve this, let's revert the commit, and add a GLib specific
workaround for the build failure.

This is safer, and works without epoll().  Fixes a build failure with
uClibc, which does not support epoll().

Replaces GLib's g_ascii_strcasecmp().

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.