core: fill in nm-types.h, clean out other headers

Clean up some of the cross-includes between headers (which made it so
that, eg, if you included NetworkManagerUtils.h in a test program, you
would need to build the test with -I$(top_srcdir)/src/platform, and if
you included nm-device.h you'd need $(POLKIT_CFLAGS)) by moving all
GObject struct definitions for src/ and src/settings/ into nm-types.h
(which already existed to solve the NMDevice/NMActRequest circular
references).

Update various .c files to explicitly include the headers they used to
get implicitly, and remove some now-unnecessary -I options from
Makefiles.

src/NetworkManagerUtils.h

@@ -26,12 +26,7 @@
 #include <stdio.h>
 #include <net/ethernet.h>
 
-#include "nm-ip4-config.h"
-#include "nm-setting-ip4-config.h"
-#include "nm-ip6-config.h"
-#include "nm-setting-ip6-config.h"
 #include "nm-connection.h"
-#include "nm-setting-private.h"
 
 gboolean nm_ethernet_address_is_valid (const struct ether_addr *test_addr);