Fix improper use of non-atomic pointer validity to report state of background search thread state

Zenity and Kdialog only support opening either files or directories
during a single invocation.

- Add AlertWindow::show() and showAsync() methods that take a MessageBoxOptions argument
  - Add NativeMessageBox::show() and showAsync() methods that take a MessageBoxOptions argument
  - Update the DialogsDemo to demonstrate the new methods
  - Deprecate AlertWindow::showNativeDialogBox() in favour of the NativeMessageBox methods
  - Pass button strings specified in MesssageBoxOptions to native dialog boxes correctly
  - Use modern TaskDialog on Windows for the native dialog box where available

See BREAKING-CHANGES.txt for more details.

When bridging 32-bit/64-bit plugins on Windows, the plugin is hosted in
an auxilliary process, and the plugin's editor is embedded into an HWND
owned by a different process (the plugin host).

Previously, the `isForegroundProcess` check would fail for bridged
plugins, because the foreground window may belong to the DAW, rather
than to the auxilliary hosting process.

This patch adds an additional check, to find whether the same process
owns both the foreground window, and the window which embeds the
PopupMenu's target component. In this case, we avoid immediately
dismissing the PopupMenu.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.

It seems like shared_from_this may not be enabled when a unique_ptr is
assigned to a shared_ptr (although it *should* be enabled when
constructing a new shared_ptr from a unique_ptr). Functions that return
objects that may need to use shared_from_this now return shared_ptr,
just to be safe.

Additionally, in some cases, shared_from_this was being called from
Thread::run after the last reference to the shared object had been
released. We now call shared_from_this during 'open', which will always
run on the message thread while at least once reference to the shared
object is alive.