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
use opacity mask to avoid warnings
Merge pull request #1061 from MahApps/955-punker76-flyout-binding-warnings

fix flyout warnings
Merge pull request #1056 from MahApps/1048-punker76-textbox-spellchecking-menu

enable spell checking context menu