Code review comment for lp:~sil2100/unity/hud_icon_bfb_fix

More than doing this in the HUD itself, probably it's just better to avoid to get an unity window as the active one in the Application manager (as we did before).

To do that just filter-out the xid's that are in nux::XInputWindow::NativeHandleList() in bamf::Manager::GetActiveWindow()