altTab: Only create one settings instance per window switcher popup

Every settings instance we create is a round-trip to the dconf
daemon, so we need to be careful of not creating them too haphazardly.

https://bugzilla.gnome.org/show_bug.cgi?id=707806
Revert "popupMenu: Make the section invisible if it has no visible children"

This reverts commit 5a0ac6c2ac5686c679988c2ca922ed758d2b19f1.

https://bugzilla.gnome.org/show_bug.cgi?id=707801
popupMenu: Fix algorithm for separator visibility to work with sections

Before, separators naively checked whether their siblings were visible
using actor visibility. However, if section actors are visible but have
no visible children, this will fail. Special-case separators when doing
visiblity checks.

https://bugzilla.gnome.org/show_bug.cgi?id=707801