Fixed: browser would scroll text fields into view when they focused.

Previously, there were two instances in which a browser would forcibly scroll a text field into view, out of Cappuccino's control:

- A text field is first responder in the key window, is partially or fully offscreen, and you click somewhere else within the same window. If the target of the click does not accept first responder, the text field is refocused.

- A text field is first responder in a non-key window, is partially or fully offscreen, and you make its window the key window. In that case the text field is made first responder and is focused.

In both cases, focusing the text field causes the browser to scroll the viewport out of Cappuccino's control such that the text field is completely onscreen.

With this commit, before a text field is focused, it is checked to ensure it is completely within the usable content rect of the platform window. If not, it refuses first responder. If the window is becoming key, the first responder is set to nil.