Clicking Nodes\Links brings them up in viewer

[Nosille]

I added the ability to mouse click on nodes and links in the graph view and have them populate slider_A and slider_B in the main view window.

[matlabbe]

That feature already existed here:

rtabmap/guilib/src/GraphViewer.cpp

Lines 1915 to 1939 in ddb9b54

	void GraphViewer::mouseDoubleClickEvent(QMouseEvent * event)
	{
	QGraphicsItem *item = this->scene()->itemAt(mapToScene(event->pos()), QTransform());
	if(item)
	{
	NodeItem *nodeItem = qgraphicsitem_cast<NodeItem*>(item);
	LinkItem *linkItem = qgraphicsitem_cast<LinkItem*>(item);
	if(nodeItem && nodeItem->parentItem() == _graphRoot && nodeItem->id() != 0)
	{
	Q_EMIT nodeSelected(nodeItem->id());
	}
	else if(linkItem && linkItem->parentItem() == _graphRoot && linkItem->from() != 0 && linkItem->to() != 0)
	{
	Q_EMIT linkSelected(linkItem->from(), linkItem->to());
	}
	else
	{
	QGraphicsView::mouseDoubleClickEvent(event);
	}
	}
	else
	{
	QGraphicsView::mouseDoubleClickEvent(event);
	}
	}

but it was on double-click. I tried your PR and it feels better with single-click. I modified the PR to use single-click now. I also kept your spin boxes next to the main slider bars, which are indeed useful. I merged the spin box with the index label, as they represented the same function:

I removed the code about "highlighting" option, which seemed not used anywhere. The border context menu option also crashed when clicking on it, so I removed it. I suggest to open another MR if there was a new feature there that could be useful to keep.

I'll merge now with those changes.