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/nm-activation-request.h

@@ -23,6 +23,7 @@
 
 #include <glib.h>
 #include <glib-object.h>
+
 #include "nm-types.h"
 #include "nm-connection.h"
 #include "nm-active-connection.h"
@@ -35,9 +36,9 @@
 #define NM_IS_ACT_REQUEST_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), NM_TYPE_ACT_REQUEST))
 #define NM_ACT_REQUEST_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), NM_TYPE_ACT_REQUEST, NMActRequestClass))
 
-typedef struct {
+struct _NMActRequest {
 	NMActiveConnection parent;
-} NMActRequest;
+};
 
 typedef struct {
 	NMActiveConnectionClass parent;