release v0.20.14

When the decoder is still starting up while we handle a SEEK, finish
the "player SEEK" immediately and re-enter the player loop, being able
to handle commands (and even cancel the pending seek).

This is the first part in a series of patches to solve the "blocking
input blocks decoder, blocks player, blocks the main thread" problem.
There are many other blocking code locations left, and the main thread
isn't non-blocking either because it waits for "seeking" to become
false.

This declaration is obsolete because MPD has switched on RTTI long
ago.

The STOP, EXIT and CLOSE_AUDIO commands are not finished here; they
are propagated to PlayerControl::RunThread() where the
outputs.Cancel() call will be done again.

Allows signalling a failed seek, and replaces several redundant
command checks after the ProcessCommand() call.

Decouple this function from player command execution.

Thsi filter doesn't make sense as manually configured plugin, because
nobody ever calls volume_filter_set() on it, making it a no-op.

These are not in filter_plugins[] and have dedicated constructors
anyway.

Supporting GCC 4.9 is becoming cumbersome as we refactor more code to
C++14.

This shouldn't be a problem for anybody, because Debian Stable
(Stretch) has version 6.3, and Ubuntu LTS (Xenial) has version 5.3.

This resolves the circular dependency between IcyInputStream and
CurlInputStream.