add icons to preset `<check>`s

[Klumbumbus]

It would be great if we could add icons to preset <check>s.

(It already works for <combo>s and <label>s)

I just added icons to the entrance and crossing combos in r10437:

In context of #11006 I would like to add the different public transport icons to the stop_position preset, see the following screenshot. That would make it much more clear for the user, which icon belongs to which tag.

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2016-06-20 02:47:43 +0200 (Mon, 20 Jun 2016)
Build-Date:2016-06-20 01:34:14
Revision:10436
Relative:URL: ^/trunk

Identification: JOSM/1.5 (10436 en) Windows 7 32-Bit
Memory Usage: 247 MB / 742 MB (110 MB allocated, but free)
Java version: 1.8.0_91-b14, Oracle Corporation, Java HotSpot(TM) Client VM
VM arguments: [-Djava.security.manager, -Djava.security.policy=file:<java.home>\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=56579, -Djnlpx.jvm=<java.home>\bin\javaw.exe, -Djnlpx.vmargs=LURqYXZhLnV0aWwuQXJyYXlzLnVzZUxlZ2FjeU1lcmdlU29ydD10cnVlAA==]
Dataset consistency test: No problems found