Only load panel corresponding to OnlyShowIn/NotShowIn

Most panel desktop files have an OnlyShowIn entry to display them
in the previous iconish G-C-C UI. Some distributions (like ubuntu) prefers
adding new panel rather than patching the existing ones to ensure that
the upstream Control Center look it preserved under a vanilla GNOME session.
This patch fix thus the new layout setting to respect the OnlyShowIn property.

https://bugzilla.gnome.org/show_bug.cgi?id=787347