Make sure we only remove view title from the directive that set it

[vstene]

When doing a page change in ui-router, the scope.on('$destroy') is called after the new view title directive has been initialized, resulting that there is no view title present.

[apparentlymart]

Thanks for this! It's a great improvement to the robustness of the title handling.

I just had a design-ish question inline. If you don't have time to work on this further then no worries... just thinking about what is the best way to represent this internal state.