Implement better resolution of window aggregate return types

Fixes https://issues.apache.org/jira/browse/OPTIQ-292

I did not touch the following code in `SqlToRelConverter`, however it might make sense to add proper type resolution here (with account of group count)

```java
  private RexNode convertOver(Blackboard bb, SqlNode node) {
...
    RexNode rexAgg = exprConverter.convertCall(bb, aggCall); // this does not take groupCount into the account
    rexAgg =
        rexBuilder.ensureType(
            validator.getValidatedNodeType(call), rexAgg, false);
```