Select Target Layer dropdown to small

[Hb---]

What steps will reproduce the problem?

1. Create two layers
2. Rename the top layer to "n" or a similar short name
3. Activate the bottom layer
4. Ctrl+M (or rightclick Merge)

What is the expected result?

See a dialog with a dropdown wide enough to show meaningful layer names.

What happens instead?

Dropdown is only three characters wide.

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

The length of the first layer name distinguishes the width of the dropdown field. This field needs an minimum width.

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2017-11-15 20:09:06 +0100 (Wed, 15 Nov 2017)
Build-Date:2017-11-16 02:31:57
Revision:13124
Relative:URL: ^/trunk

Identification: JOSM/1.5 (13124 en) Windows 7 64-Bit
OS Build number: Windows 7 Professional (7601)
Memory Usage: 654 MB / 3604 MB (294 MB allocated, but free)
Java version: 1.8.0_121-b13, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM
Screen: \Display0 1280x1024
Maximum Screen Size: 1280x1024
Dataset consistency test: No problems found

Plugins:
+ PicLayer (33822)
+ apache-commons (33668)
+ ejml (32680)
+ geotools (33380)
+ jts (32699)
+ measurement (33760)
+ opendata (33791)
+ pdfimport (33579)
+ print (33826)
+ utilsplugin2 (33742)