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.
Merge remote-tracking branch 'engine-origin/master' into translation_de

Conflicts:
	mods/default/languages/engine.be.po
	mods/default/languages/engine.de.po
	mods/default/languages/engine.el.po
	mods/default/languages/engine.es.po
	mods/default/languages/engine.fi.po
	mods/default/languages/engine.fr.po
	mods/default/languages/engine.gl.po
	mods/default/languages/engine.it.po
	mods/default/languages/engine.ja.po
	mods/default/languages/engine.nb.po
	mods/default/languages/engine.nl.po
	mods/default/languages/engine.pl.po
	mods/default/languages/engine.pot
	mods/default/languages/engine.ru.po
	mods/default/languages/engine.sk.po
	mods/default/languages/engine.sv.po
	mods/default/languages/engine.uk.po