Treat "about:blank" as a normal empty page

The "about:blank" url is not shown to user. Instead,
we are showing the empty text.
[sailfish-browser] Save empty default home page as "about:blank"
Merge pull request #215 from rainemak/emptyPageFix

Fix empty page handling
Cleanup url and title update sequence

In normal navigation cases, url and title are written to database
but callback is not triggered. DBWorker emits tabChanged
from DBWorker::goBack or DBWorker::goForward.

Conflicts:
	src/declarativetabmodel.cpp
Align tabmodel unit tests to previous commit