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_strncasecmp().

Replaces GLib's g_ascii_strcasecmp().

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

Fixes busy loop when USB sound device gets unplugged (Mantis bug
#3824).

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Simplify the API, don't use GLib specific integer type.