DBViewer: Add mode to add manually multiple loop closures without optimizing graph right away

[matlabbe]

Related to #1368

If the graph is huge, it is tedious to add manually loop closures to fix some parts of the map when the graph is optimized after each "add" and it takes a lot more than 5 sec. The graph also re-zooms on the full map so we loose track of the current section we are looking at. It could be nice to disable auto-optimization after each action modifying the graph so that we can do many edits before doing the long optimization.

Note that the auto-optimization when adding loop closures is there to be able to detect bad loop closures. We could disable auto-optimization only if RGBD/OptimizeMaxError is 0 (which re-optimization doesn't matter as the check won't be done).