The biggest new feature in this commit is the addition of
NativeMessageBox::scopedAsync and AlertWindow::scopedAsync, both of
which return an instance of ScopedMessageBox that will hide the message
box in its destructor.

The code for displaying modal dialogs on Windows has also been updated.
Now, the dialog itself is run from a new thread with its own message
loop. This means that when the dialog is dismissed, the background
thread can be joined safely. In plugins, this means that there's no
danger of the plugin view being destroyed from within the message box
runloop, for example.

On Windows, broughtToFront may be called during the destructor of
VSTPluginHost, adding the window back into activeVSTWindows, leading to
a heap-use-after-free in vstHookWndProc.

This change means that broughtToFront will only reorder
activeVSTWindows; it will never add a window that's not already in the
list.

JUCE no longer depends on built artifacts of the AAX_SDK

The OpenGL renderer uses ColourGradient::createLookupTable to generate
gradient textures. However, the tweening method used was different to
the tweening used by CoreGraphics gradients, and by the software
renderer.

Gradient tweening is now computed using non-premultiplied colours, to
ensure consistency between gradients rendered using OpenGL, and with
other renderers.

AUv2 plugins on Arm that are hosted out-of-process (e.g. in Logic 10.7)
can sometimes crash due to endlessly recursing through becomeKeyWindow.
This tends to happen when displaying a secondary window in a plugin,
e.g. an AlertWindow, then clicking on a secondary app, then clicking
back on the AlertWindow.

To avoid this case, we check that the peer isn't already key before
calling makeKeyWindow.

Unfortunately, we can't use isKeyWindow to avoid the recursion because
this may not return true until after becomeKeyWindow has returned.

The previous method for finding the dialog's owner was ineffective, and
it was still possible for other windows to obscure the file picker in
many cases. Using GetActiveWindow seems to produce the expected
behaviour.

Without this VoiceOver will iterate over menu items left to right first,
and iteration order will be affected by whether the PopupMenu gets
broken up into multiple columns due to not enough screen space.

Without this event Components such as Button would remain in a hover
state indefinitely after being clicked on a touch screen. The simulated
event matches the behaviour of the other platforms.

CGContextGetClipBoundingBox sometimes returns a 'null' rect. When it
does, the following logic will get stuck in the while loop drawing
individual tiles over and over again.

Opening some JUCE VST3s in FL Studio's patcher can cause crashes because
of data races on objects accessed from setActive() and process().

According to the VST3 specification, these functions should never be
called simultaneously, so I think this is a bug in FL Studio.

This allows paths that are prefixed with environment variables to behave
as expected. This is useful when scanning the default LV2 locations in
the AudioPluginHost on Windows.

Some hosts (e.g. REAPER) will attempt to instantiate plug-ins with 64
active channels, and JUCE can't represent this layout. In this case,
failing to convert between VST3/JUCE layouts is not a logical error, so
asserting is not appropriate.

Fixes #1118