Add support for FlowPanels to have custom tags.

This is useful for translating HTML code to UiBinder, where a "panel"
needs to have, per the mockup's HTML/CSS, an "li" tag or "p" tag.

Previously this required using HTMLPanel, but HTMLPanel doesn't
implement InsertPanel, which has a useful add(Widget, index) method.

Change-Id: Ic43bba2c115c7b531b75ed9f7a5d81371e231893