Change icons in settings app

Symbolic icons have issues with some gtk2 themes where they don't
get colored properly. Additionally some themes don't have network-fs
or gnome-dev-network so change to gtk-network.
Fix a typo
Add versioned help when a GtkPlug

This patch calls the versioned online help docs when xfdesktop-settings
is embedded as a GtkPlug.
update potfiles for file rename
Use the macro in the .in.in desktop file