warn when closing relation editor window with x top right

[Klumbumbus]

What steps will reproduce the problem?

1. open relation editor
2. do some changes
3. press x top right of the window

What is the expected result?

question if you want to cancel the changes like if you press cancel

What happens instead?

window closes and the changes are lost without a warning

Please provide any additional information below. Attach a screenshot if possible.

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2015-11-21 22:45:33 +0100 (Sat, 21 Nov 2015)
Build-Date:2015-11-22 02:32:57
Revision:9053
Relative:URL: ^/trunk

Identification: JOSM/1.5 (9053 de) Windows 7 32-Bit
Memory Usage: 476 MB / 742 MB (88 MB allocated, but free)
Java version: 1.8.0_66, Oracle Corporation, Java HotSpot(TM) Client VM
VM arguments: [-Djava.security.manager, -Djava.security.policy=file:C:\Program Files\Java\jre1.8.0_66\lib\security\javaws.policy, -DtrustProxy=true, -Djnlpx.home=<java.home>\bin, -Djnlpx.origFilenameArg=C:\Program Files\josm-latest-bla.jnlp, -Djnlpx.remove=false, -Djava.util.Arrays.useLegacyMergeSort=true, -Djnlpx.heapsize=256m,768m, -Djnlpx.splashport=50583, -Djnlpx.jvm=<java.home>\bin\javaw.exe, -Djnlpx.vmargs=LURqYXZhLnV0aWwuQXJyYXlzLnVzZUxlZ2FjeU1lcmdlU29ydD10cnVlAA==]
Dataset consistency test: No problems found

Plugins:
- AddrInterpolation (31772)
- DirectDownload (31772)
- DirectUpload (31772)
- HouseNumberTaggingTool (31772)
- OpeningHoursEditor (31772)
- PicLayer (31772)
- RoadSigns (31772)
- apache-commons (31772)
- buildings_tools (31772)
- download_along (31772)
- editgpx (31772)
- imagery_offset_db (31772)
- log4j (31772)
- measurement (31772)
- photo_geotagging (31772)
- photoadjust (31772)
- public_transport (31772)
- reltoolbox (31772)
- reverter (31772)
- tag2link (31603)
- tageditor (31772)
- tagging-preset-tester (31772)
- terracer (31772)
- turnlanes (31772)
- turnrestrictions (31772)
- undelete (31772)
- utilsplugin2 (31772)
- wikipedia (31772)

[simon04]

In 9511/josm:

fix #12131 - Confirm closing relation editor window with x top right

[simon04]

In 9512/josm:

see #12131 - Pressing escape in closing relation confirmation should not discard the changes and close the editor