clip relative to widget popups to window

before this the relativeToWidgetPopupPositioner would position the popup in the following way:

if there is enough space to the right of the widget it would position the popup on the left edge of the widget.

else if there is enough space on the left then popup is positioned on the right edge of the widget.

But if there is not enough space to the left or the right then the popup is positioned on the left edge of the widget and the width of the page is increased to accommodate it.

I think the last option is undesirable, so now there is a clipToWindow field that is true by default.  And when there is no space then the popup is positioned on the left edge of the screen, so it is less likely that the page will need to expand to accommodate it.

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I also got rid of the excessive commentary that was copied across from gwt's relativeToWidget position code.