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

Not only those which are "ready".

Prepare for adding more code here.

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.

Allows forward declaration.

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.

By making each SocketMonitor keep track of its ready flags, this
removes a lot of overhead from EventLoop::RemoveFD().

This just happened to break the Windows build because of the
`GetObject` macro in `windows.h`, so I added a kludge to
PollResultGeneric.hxx.

Relax the assertions.  This is necessary if BlockingCall() is used
before the thread is started.

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>