Obsoletes Tag::BeginAdd() and the complicated "bulk add" code.

This is not a C++ class and must not be copied.

Move to TagSettings.c and use C99 initializers.

Allow using the Tag.cxx library without the Config library.

Migrate the remaining callers to FatalError().

Replaces struct playlist_provider.

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

Reduce header dependencies.

Reduces header dependencies.

Replaces GLib's GError.

Remove all #ifdefs.  Old versions of libpulse are not being tested,
and thus I'm removing support.

Fixes build failure on GLib < 2.32.

New GLib versions don't fail.

Omitting it has the same effect as returning false unconditionally.

Temporary hotfix until we have integrated avahi into our new event
loop.

Implement an event loop without GLib.

Fixes regression from commit 018f4155.