add 'set working dir as project dir' button

This PR adds a small icon to the left of the Project button in the global toolbar that sets the current directory to the project directory.

![screen shot 2014-11-22 at 4 03 18 pm](https://cloud.githubusercontent.com/assets/1976582/5156117/1f732cb4-7261-11e4-90e8-e1e6db77d999.png)

The icon will be hidden if there is no active project. If we take this, we might want to use a different icon as well.
This command handler belongs in a more global scope (i.e. the "Workbench" class).