Add initial unit tests for Tab

Run like:
> ssu ar mer-tools
> pkcon refresh && pkcon install testrunner-lite
> testrunner-lite -f /opt/tests/sailfish-browser/auto/tests.xml -a -v -o /tmp/results.xml

I'd assume that it is easier to review from commit list. This PR includes declarativetab related changes that have been done for newWebView branch, I just picked the diff of declarativetab.cpp/h to this PR.