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-active-connection.c

@@ -28,6 +28,7 @@
 #include "nm-device.h"
 #include "nm-settings-connection.h"
 #include "nm-manager-auth.h"
+#include "nm-auth-subject.h"
 #include "NetworkManagerUtils.h"
 
 #include "nm-active-connection-glue.h"