Add showIf to group actions, and new separator group action.

showIf="pythonFuncName" calls the passed python function with the current selection and
filters the actions 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.

This also adds the XML group action items <cardseparator /> and <groupseparator /> to add separators between card and group items respectively.