Provide option allowing to choose whether URLs should be spell checked.

When the spell-check find an error in a TextBox, the wrong word should be underlined with a red curly line.

Improvements:  

1.Allow loading of dialogs (SpellCheckAllAtOnce and SpellCheckWordByWord) different than the default ones through MEF. Use-cases: set different Width and Height, hide buttons (e.g. AddToDictionary), etc.  

2. When the custom control for spell checking contains several TextBoxes, allow spell checking of them in turns, instead of opening a different spell checking dialog for each. 

3. Make the TextBox spell checker generic, so that controls that inherit from TextBox can be spell checked out of the box. 

This item has been declined and separated into different ones. Please, follow them:
- https://feedback.telerik.com/Project/143/Feedback/Details/245921
- https://feedback.telerik.com/Project/143/Feedback/Details/245919
- https://feedback.telerik.com/Project/143/Feedback/Details/245917

Make the TextBox spell checker generic, so that controls that inherit from TextBox can be spell checked out of the box. 

When the custom control for spell checking contains several TextBoxes, allow spell checking of them in turns, instead of opening a different spell checking dialog for each.