Add checkbox to 'merge selected' dialog box to remove merged objects from source layer

[AndrewBuck]

I have been using JOSM to selectively merge in lots of town names from the GNS database in Mali. Since I want to manually review each one I keep the source data as a separate layer and then do 'merge selected' on one (or a few) nodes at a time to merge them into the OSM layer that I subsequently upload. Currently everytime I do a merge of some objects I also then have to hit delete to remove these objects from the original source layer otherwise they end up in both layers (I want them removed from the source layer so I can track which ones have been merged in already).

In order to make this a lot quicker and less prone to error, it would be nice to have a checkbox in the 'merge selected' dialog that pops up asking if you want to also remove the merged objects from the source layer (leaving the box unchecked would retain the current behavior). The merged objects (if the box is checked) should just be 'purged' from the source layer, i.e. they should not be marked marked for deletion, just removed from the layer. For the text for the checkbox I would suggest something like "Purge objects from source layer" or "Remove objects from source layer" so that is clear to the user that they will not be deleted, just removed from the original layer.

Finally, the checkbox should remember whether it is checked or not (I would default to unchecked until the user checks it and remember the state from then on). Having to check the box each time would be no easier than hitting delete after the merge, which is the current use scenario.

[Don-vip]

Nice idea, I often do the same when integrating open data.