fix #8426: add_tags in remote control: better changed tags detection,
fix EDT violations and NPE when layer is closed while adding tags
added GuiHelper.executeByMainWorkerInEDT to execute GUI-related tasks sequentially from non-EDT threads

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5790 0c6e7542-c601-0410-84e7-c038aed88b3b