QWinThumbnailToolButton: use Q_SLOTS in public header

Change-Id: Ife9dedcadb5c3bca4ce7ed02bc7038b6d2a1dc2f
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Add headersclean test

Change-Id: I295d2592c99b721617ce1fd74fd519435f49d861
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>