... and make TimerEvent a type alias for FineTimerEvent (i.e. swap
names).

Similar to commits 1686f4e8 and
30a5dd26

Not for MPD, but for other applications which might want to copy its
event loop, but do not need multi-threading.

Instead of using this as a base class implementing a virtual method,
the new class IdleEvent can be used as a variable, decoupling
IdleMonitor's internal state from the derived class.

This is similar to commit 30a5dd26
which refactored TimeoutMonitor to TimerEvent.

The event library uses various libnet.a classes,
e.g. SocketDescriptor.

Prevents compiling those sources many times, once for each debug
program using it.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!