Layout widgets don't initialize correctly inside hidden node

[bill]

TabContainer inside a Dialog won't work correctly because when TabContainer.startup() is called, the TabContainer is hidden and thus functions like dojo.getMarginBox() returns 0. Same thing for other cases, like having an Editor inside a hidden tab.

Doug found that you can connect to an onload event on a dom node that isn't triggered until the node is displayed on the screen, so maybe we can use that technique to do resizing as the dialog is displayed.

[bill]

Dup of #4169.