Core API: add index modes afterwards (#649).

One can add index modes to types afterwards, e.g. in a migration.
Existing topics/associations are indexed subsequently.

Use the `Type`'s new `addIndexMode()` method.
Example:
{{{
public class Migration2 extends Migration {

    @Override
    public void run() {
        dms.getTopicType("ka2.geo_object.name").addIndexMode(IndexMode.FULLTEXT_KEY);
    }
}
}}}

BREAKING CHANGE

`Type` API:
{{{
void setIndexModes(List<IndexMode> indexModes) -> void addIndexMode(IndexMode indexMode)
}}}

`TypeModel` API:
{{{
void setIndexModes(List<IndexMode> indexModes) -> void addIndexMode(IndexMode indexMode)
}}}

See #649.