Fix insensitive styling for popup menu items

Fix insensitive styling for popup menu items

The selector for insensitive popup menu items was wrong (a PopupMenuItem is
a ShellGenericContainer, not a StButton). Fixing it showed that previous
:insensitive tracking was manual for a reason: we have many items that are
not reactive, but don't want the insensitive styling (for example those in
the battery menu).
Fix it by adding a new style-class, popup-inactive-menu-item, that is added
to all new PopupMenuItems that are not activatable.

https://bugzilla.gnome.org/show_bug.cgi?id=683988