bug: Nested popups do not release backdrop

[jbollacke]

Type: bug

Platform: all

Once one opens a nested popup (see http://codepen.io/anon/pen/ZYgLVw) the backdrop is not fully released and screen will stay dimmed.

1. Click on button "Open Popup" => backdrop dims the screen, popup no 1 opens
2. In the popup no. 1 click on "OK" => popup no1 closes, popup no2 opens
3. In the popup no. 2 click on "OK" => popup no2 closes

I expect the backdrop to be released and "un-dim" the screen again.

Note: IIRC this issue already popped up once upon a time. Take a look at my comment on 5658a4d. Looks like popup/backdrop component really needs some more test coverage.