Fix real tabs count to work correctly over browser restarts

Fix PR fixes live tabs count not be only based on runtime value. Now will always virtualize tabs that are in index greater than liveTabCount would allow. Previous lastTabId() way only work during runtime (totally broken), after browser restart lastTabId could have been pointing to e.g. tab number 10. Thus, allowed creating 9 tabs.

This commit exposes liveTabCount to QML and that could be bound
to e.g. a settings value.

PR adds unit tests for live tab count.