Prevent user from selecting the same layer for Subject and Reference

[bagage]

When using the same layer for both Subject and Reference, Conflation computes things a bit oddly - when you would expect everything to match with 0m of distance and score of 1, it actually ends up with strange results.

I believe it does not make sense to use the same layers for both entries in anycase, so here's a small patch preventing it.

Additional information below.

Build-Date:2017-03-24 10:36:25
Revision:11772
Is-Local-Build:true

Identification: JOSM/1.5 (11772 SVN en) Linux Debian GNU/Linux 9.0 (stretch)
Memory Usage: 829 MB / 1760 MB (611 MB allocated, but free)
Java version: 1.8.0_121-8u121-b13-4-b13, Oracle Corporation, OpenJDK 64-Bit Server VM
Screen: :0.0 1920x1080
Maximum Screen Size: 1920x1080
Java package: openjdk-8-jre:amd64-8u121-b13-4
Java ATK Wrapper package: libatk-wrapper-java:all-0.33.3-13
Program arguments: [${HOME}/CL096-CLERIEUX/test-small.osm]
Dataset consistency test: No problems found

Plugins:
+ PicLayer (33148)
+ conflation (0.5.1)
+ jts (32699)
+ rex (26)
+ utilsplugin2 (33182)

Last errors/warnings:
- W: No configuration settings found.  Using hardcoded default values for all pools.