Add an option that automatically generates an export header for a Qt
module. The header contains only Q_DECL_EXPORT/Q_DECL_IMPORT related
content, so it's not a full replacement of 'global' header files.
Task-number: QTBUG-90492
Change-Id: I250d1201b11d4096b7e78e61cbf4565945fe6517
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>