Fix history search to return distinct results

PR contains 5 commits that are more less bound together.

- Unit tests for declarativetabhistory
- History database fix
- Cleaning up DeclarativeHistoryModel to use only global history. DBWorker has still tab history related methods as I thought that it could be handy if we'd like to improve tab navigation further and take into account the whole back/forward navigation history. Currently we only know previous and next link statuses.
- Due to history database fix DeclarativeTabModel::currentTabId became obsolete -> moved to unit tests
- Updated DeclarativeTabModel to use new TestObject base class
Might be easier to review form commits.