Split the content of libnm/nm-vpn-editor-plugin.h and
move NMVpnEditorPlugin to libnm-core/nm-vpn-editor-plugin.h.
VPN plugins allow us to extend functionality about VPNs.
This can be also useful for NetworkManager core, hence
move that part to libnm-core.
The name NMVpnEditorPlugin is slightly misleading but not completely
wrong. The "editor" part stands no longer for bringing nm-applet
functionality (alone), but enable general VPN functionality in
the client.
Especially because we already have NMVpnPluginOld with a different
meaning (i.e. a base class of the plugin server implementation).
