Fix spellchecking for macOS

The change fixes spellchecking to work on macOS.
A new WebEngine configure option is available to allow spellchecking on
macOS to use either Hunspell like the other platforms, or the native
spellchecker that comes with the OS. The default is to use Hunspell.

Task-number: QTBUG-53135
Change-Id: I3e45b2e0d728b1bf2659c35f3d0a042b0ecd6239
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>