Fix real tabs count to work correctly over browser restarts

This commit exposes liveTabCount to QML and that could be bound
to e.g. a settings value.
Merge pull request #70 from rainemak/liveTabCountFix

Fix real tabs count to work correctly over browser restarts
Move tabModel property to C++ side
Rename TabCache to WebPages

Ownership of TabCache will be moved to DeclartiveWebContainer
in following commit(s).
Move WebPages to DeclarativeWebContainer's ownership