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.

Prepare for migrating away from GLib.

Unsigned and portable.

Handle timeout internally.

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

Remove a GLib dependencies from class EventLoop and DatabaseLock.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

Track which thread runs the EventLoop and provide a check whether
we're currently inside.

Prepare for adding more code.