TabSheetElement now works with tabs without a caption (#14434).

- The changes affect the methods getTabCaptions, openTab and closeTab. A
test for each method has been added.
- Added methods getTabCount(), openTab(index) and closeTab(index) and
tests for these methods.

Change-Id: I86f0b1c5a1786521648aa97932bd8787c63c4428