webadmin,userportal: Fix popups after engine restart & relogin

This patch ensures that all application popups (both non-singleton
and singleton) can be shown again after auto-logout and re-login
due to engine restart, without having to reload WebAdmin page.

RootPresenter's popupChildren list should no longer get polluted
after each auto-logout, which would cause conflicts when trying
to reveal given application popup.

Change-Id: Icd934c9ec6129b0b45224ddb08a5c41b070a60bc
Bug-Url: https://bugzilla.redhat.com/968362
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>