add attached property UseSpellCheckContextMenu
finished changes

- rename property to IsSpellCheckContextMenuEnabled
- don't show separator if suggestions are empty
- raise InvalidOperationException if element isn't a TextBoxBase
- add property to TextBox and RichTextBox at main window example
Merge pull request #1056 from MahApps/1048-punker76-textbox-spellchecking-menu

enable spell checking context menu