[license] Next depends not just on the checked box

- Toggling the checkbox could disable the next button
   because only the checked-state was used, instead of
   the next-is-enabled-if-everything-is-optional member variable.

FIXES #1271

src/modules/license/LicensePage.cpp

@@ -211,5 +211,5 @@ LicensePage::checkAcceptance( bool checked )
     {
         ui->acceptFrame->setStyleSheet( "#acceptFrame { padding: 3px }" );
     }
-    emit nextStatusChanged( checked );
+    emit nextStatusChanged( m_isNextEnabled );
 }