Fixed: delete methods erased entire field

Previously, the deleteForward:, deleteBackward: and delete: methods in CPTextField called setStringValue, which would consult a formatter, and if the formatter failed all of the text would be deleted.

Now the code has been refactored such that when one of the delete methods is invoked via a key event, the formatter is not used and the DOM event is used, which results in the expected behavior.

A bug in checking the selected range was also fixed in deleteForward:.

The CPFormatter test was updated with buttons to call the delete methods directly to ensure they work correctly when not invoked from a key event.

Closes #1997