Fixed missing translation strings on loading campaign save

- campaign header now contains text container and stores campaign texts
- map header now contains text container instead of inheriting it
- moved text container registration logic to a wraper class
- fixed registration of copied text containers (from copied map header)

lib/campaign/CampaignState.cpp

@@ -169,6 +169,11 @@ const CampaignRegions & CampaignHeader::getRegions() const
 	return campaignRegions;
 }
 
+TextContainerRegistrable & CampaignHeader::getTexts()
+{
+	return textContainer;
+}
+
 bool CampaignState::isConquered(CampaignScenarioID whichScenario) const
 {
 	return vstd::contains(mapsConquered, whichScenario);