The old design had us checking isActive inside our audio callbacks, and
also modifying isActive from prepareToPlay(), and releaseResources(). To
avoid race conditions, and to ensure that isActive actually reflects the
activation state of our plugin, we need to lock in these places. If we
don't lock, there's a chance that other threads will observe isActive to
be true while the plugin is not actually active (for example).

If you're not convinced, imagine this scenario:
- The message thread calls prepareToPlay. The plugin is activated.
- The audio thread starts calling processBlock, and gets as far as the
  isActive check. The plugin appears active, so we continue into
  processBlock.
- At the same time, the message thread calls releaseResources(). The
  processBlock call is still in progress, but the message thread is
  simultaneously making calls to setProcessing() and setActive(), which
  is a race.

Normally, it'd be up to the host of the AudioProcessor to ensure that
prepareToPlay() isn't called at the same time as processBlock().
However, VST3 plugins can request a restart at any time from the UI
thread, requiring us to call prepareToPlay() even while processBlock()
is running.

Projucer:  Fixed an assertion caused by the project content component trying to grab keyboard focus when not visible

Accessibility:  Added VoiceOver (macOS) and Narrator (Windows) accessibility screen reader support to juce_gui_basics

Previously, IEditController parameter indices were being used to index
into the AudioProcessor parameter array, but these parameter indices are
not guaranteed to point to the same parameter (parameter groups may
cause reordering on JUCE's side). Now, we use the IEditController
indices universally.