Fix opening context menu when item is locked

Description

Fix opening context menu when item is locked

When trying to ignore an event, true must be returned, false means the event was handled and therefore QGraphicsscene does not propagate the event to other items. See graphicsscene.cpp

    for (QGraphicsItem *item : items) {
        contextMenuEvent->setPos(item->d_ptr->genericMapFromScene(contextMenuEvent->scenePos(),
                                                                  contextMenuEvent->widget()));
        contextMenuEvent->accept();
        if (!d->sendEvent(item, contextMenuEvent))
            break;
        if (contextMenuEvent->isAccepted())
            break;
    }

Conformity

• Changelog entry
• Unit tests Small change
• Update INSTALL
• Downport
• Fixes: #829 (closed)

When external dependencies are removed

• Reporting to ...