Bug 886329 - drop-down list in Jetpack add-on breaks entire UI in Aurora (23.0a2)

- Ignored `popupshowing` events are not emitted by panel
- Added unit test
- Added `ready` helper function
Merge pull request #1221 from ZER0/panel/886329

fix Bug 886329 - drop-down list in Jetpack add-on breaks entire UI in Aurora (23.0a2) r=@gozala