GroupStyleProvider is never invoked for generating styles

[dtsaryov]

See a topic on support forum: link

Description of the bug or enhancement

Upd.

GroupTable has GroupStyleProvider for generating group row styles. But it is never invoked.

Steps to reproduce

1. For groupTable set GroupStyleProvider:

@Inject
private GroupTable<Test> testsTable;

@Subscribe
public void onInit(InitEvent event) {
    testsTable.setStyleProvider(new GroupTable.GroupStyleProvider<Test>() {
        @Nullable
        @Override
        public String getStyleName(GroupInfo info) {
          return "group-test";
        }
        @Override
        public String getStyleName(Test entity, @Nullable String property) {
            return null;
        }
    });
}

2. Make some column grouped

ER
In the Browser devTools we should see that the group row has "group-test" selector.
AR
Selector is not added.

Test project on 7.2-SNAPSHOT: dev-gtable.zip