Fix for autocomplete interfering with variable naming - MAGN-5292

Allow auto-insertion of highlighted autocomplete item only on certain keys:
Tab
Enter
Dot

@Benglin please review

Again I don't have UI test cases for autocomplete as of yet.
If it's 1982, I would have said four property accessors, four array indexers, and go on to suggest the following:

```csharp
switch(e.Text[0])
{
    case '\t':
    case '.':
    case '\n':
    case '\r':
        completionWindow.CompletionList.RequestInsertion(e);
        break;
}
```

But fortunately for you, it ain't 1982 no more. So LGTM!