Add a context menu to the output window with Copy and Select all buttons.

Fixes issue #3265.
Do you mind changing `OnOutputCopy` and `OnOutputSelectAll` to simple inline delegate methods:
```
miCopy.Click += (o,a) =>  Clipboard.SetText(_outputWindow.SelectedText ?? string.Empty);
miSelectAll.Click += (o,a) => _outputWindow.SelectAll();
```
This is just much simpler to understand and eliminates a bunch of plumbing.