nm: Add a widget for setting a password

It was only possible to set the password from the authentication dialog,
which is not ideal; as it requires a connection attempt.

This adds an input entry along with a primary icon from libnma/libnm-gtk
which allows selecting the backend and flags for the password (system, session
agent, always ask or empty).

src/frontends/gnome/properties/Makefile.am

@@ -19,19 +19,23 @@ common_CFLAGS = \
 
 libnm_vpn_plugin_strongswan_la_CFLAGS = \
 	$(LIBNM_CFLAGS) \
+	$(LIBNMA_CFLAGS) \
 	$(common_CFLAGS)
 
 libnm_strongswan_properties_la_CFLAGS = \
 	-DNM_STRONGSWAN_OLD \
+	$(LIBNM_GTK_CFLAGS) \
 	$(LIBNM_GLIB_CFLAGS) \
 	$(common_CFLAGS)
 
 libnm_vpn_plugin_strongswan_la_LIBADD = \
 	$(GTK_LIBS) \
+	$(LIBNMA_LIBS) \
 	$(LIBNM_LIBS)
 
 libnm_strongswan_properties_la_LIBADD = \
 	$(GTK_LIBS) \
+	$(LIBNM_GTK_LIBS) \
 	$(LIBNM_GLIB_LIBS)
 
 libnm_vpn_plugin_strongswan_la_LDFLAGS = \