Use isTradable and fix issue 1392

This change make it possible to sell spell scrolls in Artifacts Merchant.

client/widgets/CArtifactHolder.cpp

@@ -105,7 +105,7 @@ void CArtPlace::clickLeft(tribool down, bool previousState)
 	{
 		if(down)
 		{
-			if(ourArt->artType->id < 7) //War Machine or Spellbook
+			if(!ourArt->artType->isTradable()) //War Machine or Spellbook
 			{
 				LOCPLINT->showInfoDialog(CGI->generaltexth->allTexts[21]); //This item can't be traded.
 			}

client/windows/CTradeWindow.cpp

@@ -1459,8 +1459,7 @@ void CAltarWindow::showAll(SDL_Surface * to)
 
 bool CAltarWindow::putOnAltar(CTradeableItem* altarSlot, const CArtifactInstance *art)
 {
-	int artID = art->artType->id;
-	if(artID != ArtifactID::SPELL_SCROLL && artID < 7) //special art
+	if(!art->artType->isTradable()) //special art
 	{
         logGlobal->warnStream() << "Cannot put special artifact on altar!";
 		return false;
@@ -1478,7 +1477,7 @@ bool CAltarWindow::putOnAltar(CTradeableItem* altarSlot, const CArtifactInstance
 	}
 
 	int dmp, val;
-	market->getOffer(artID, 0, dmp, val, EMarketMode::ARTIFACT_EXP);
+	market->getOffer(art->artType->id, 0, dmp, val, EMarketMode::ARTIFACT_EXP);
 
 	arts->artifactsOnAltar.insert(art);
 	altarSlot->setArtInstance(art);

server/CGameHandler.cpp

@@ -2971,7 +2971,7 @@ bool CGameHandler::sellArtifact( const IMarket *m, const CGHeroInstance *h, Arti
 	const CArtifactInstance *art = h->getArtByInstanceId(aid);
 	if(!art)
 		COMPLAIN_RET("There is no artifact to sell!");
-	if(art->artType->id < 7)
+	if(!art->artType->isTradable())
 		COMPLAIN_RET("Cannot sell a war machine or spellbook!");
 
 	int resVal = 0, dump = 1;