Show VCS topic in window title

Project tree root is not visible most of the time. Window title is more
useful.

Also shows topic for *current* open file rather than the project
(supporting submodules and externally opened files)

Change-Id: Id21be09c27beca1b0d37674513121a0e0e995feb
Reviewed-by: Eike Ziller <eike.ziller@digia.com>