Fixed: scrolling CPTextField to visible would cause unexpected behavior

Previously, when a text field became the key responder, first it was scrolled to visible, then a check was made to see if the text field was completely within the platform window bounds. The scrolling to visible would unfortunately cause problems because of the interaction between the blur handler in text fields and controls that can be clicked without becoming first responder.

This commit completely forbids text fields from becoming first responder if they are not completely within the platform window. That eliminates any possibility of unwanted browser scrolling of the text field.