Recreate pot and po files

Although we have already string freeze, we need to recreate the
translation files again.

* These changes remove the 'Ok', instead we will only have 'OK'
  One way of spelling removes the burden of translations.

* We introduced 'Trade' as engine string. This is most often already
  translated, so it was just uncommented most of the time.
translation-de: shorten mouse button names.
xgettext is utf8 safe now
Translations: Bump version number to 0.18
Merge remote-tracking branch 'mainline/master'

Conflicts:
	mods/default/languages/engine.pl.po
	mods/default/languages/xgettext.py
Update version number in pot and po files.
Fix flavor text for items in xgettext tool.

Fixes #492.
Merge remote-tracking branch 'mainline/master'

Conflicts:
	mods/default/languages/engine.ja.po
Allow map events to trigger music changes

Closes #494
Merge remote-tracking branch 'mainline/master'
Merge pull request #496 from dorkster/music

Allow map events to trigger music changes
Merge pull request #495 from dorkster/bugfixes

Only show one tooltip context at a time
Merge pull request #484 from stefanbeller/master

Recreate pot and po files