[patch] Fix parsing note hiding events

[ToeBee]

I just tried to open up the latest planet note dump and hit an exception while parsing. The problem was with this note: ​http://www.openstreetmap.org/note/233775

It was mistakenly hidden which generated a comment element with a "hidden" action which JOSM doesn't know about. Interestingly, there is no unhide comment, just a reopen. The full note XML looks like this:

<note id="233775" lat="48.2411985" lon="-122.3744820" created_at="2014-08-31T17:13:29Z" closed_at="2015-09-06T23:35:14Z">
  <comment action="opened" timestamp="2014-08-31T17:13:29Z" uid="7247" user="goldfndr">Jump Start Espresso | 26930</comment>
  <comment action="hidden" timestamp="2015-09-06T23:34:26Z" uid="355617" user="pnorman"></comment>
  <comment action="reopened" timestamp="2015-09-06T23:34:38Z" uid="355617" user="pnorman"></comment>
  <comment action="closed" timestamp="2015-09-06T23:35:14Z" uid="355617" user="pnorman">mapped, but inadvertently hid the note</comment>
</note>

Anyway, here is a patch to add the "hidden" option to the enum in NoteComment.java