Make bonus stacking configurable + fix duplicate propagation/inheritance (#433)

Addresses several related problems:
* Propagation / unpropagation of duplicate bonuses is inconsistent, causing bugs
* Duplicate bonuses never stack, which is not always intended behaviour (e.g. multiple copies of resource generating artifacts)
* Different bonuses always stack, which is not always intended behaviour (e.g. Angel + Archangel morale bonuses)

This is addressed as follows:
* Duplicate bonuses are never eliminated during propagation/inheritance.
* Unpropagation eliminates only a single copy of duplicated bonus
* Bonus receives a new field stacking that determines stacking behaviour:
* * empty string = no stacking with duplicates (default)
* * "ALWAYS" = stacks with duplicates & everything else
* * some other value = no stacking with bonuses with same stacking value
Also Morale/Luck window now hides non-stacking bonuses.

test/mock/mock_BonusBearer.cpp

@@ -29,7 +29,7 @@ const TBonusListPtr BonusBearerMock::getAllBonuses(const CSelector & selector, c
 {
 	if(cachedLast != treeVersion)
 	{
-		bonuses.eliminateDuplicates();
+		bonuses.stackBonuses();
 		cachedLast = treeVersion;
 	}