Fixed: CPTextField deleteBackward: leaving 1 character.

Previously if - CPTextField `deleteBackward:` was invoked without it being a browser backspace key event for an actively edited text field, the first character in the current selection would not be deleted.

This fix makes it so that `deleteBackward:` works properly from any caller. The fix also undoes a mistake in a recent previous commit which prevented the text field from visually updating after deletion.