Deprecate Qt Audio Engine and plugin APIs in Qt Multimedia

In preparation for removing some parts of Qt Multimedia which we should no longer support, we should make sure they are deprecated in Qt 5.14.

To start with, this includes:

• Qt Audio Engine
• Plugin API

Qt Audio Engine is the 3D audio API (wrapper on OpenAL). It is not working correctly in its current state, so it is unlikely that this is really in use. Since OpenAL is a cross-platform, open source API in itself, the benefit of providing wrappers around it in Qt is limited.

The plugin APIs can remain in Qt, but they should not be public APIs since this puts a limit on what types of changes we can make, and binary compatibility is not really as relevant for plugin developers. They should be moved into private headers in Qt 6 with the appropriate warnings in place. In Qt 5 time span, they should be marked as deprecated to give a heads-up to users who may be using them.