Add showIf param to group actions

This calls the python function passed with the current selection and
filters its visibility in the context menu by the boolean result.

I had to make the OCTGN scripting engine return the value of a python
call.  I first added the Microsoft.Bcl.Async package dependency to add use
of async/await language features.  I then use this to make sure the
potentially networked script engine does not block the UI while filtering
actions.

![show if](https://cloud.githubusercontent.com/assets/354226/3696981/a85a1694-139a-11e4-9ad9-63a4257fcf3a.gif)