alerts popping up over last map

[jrobinso]

When presenting a message via igv.presentAlert the box pops up over the last "browser" panel. It should probably popup centered over the outer container of all panels. This might not be ideal, but is better than the last one. Alternatively using the first browser panel as the parent would be more predicatable.

[jrobinso]

@turner ideally general alerts should come up centered on the screen. Not any particular component, and not centered on the scrollable page document. If this is really hard we might skip it and live with current behavior.