Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

editor/fileserver/editor_file_server.cpp

@@ -31,7 +31,7 @@
 #include "editor_file_server.h"
 
 #include "../editor_settings.h"
-#include "io/marshalls.h"
+#include "core/io/marshalls.h"
 
 //#define DEBUG_PRINT(m_p) print_line(m_p)
 //#define DEBUG_TIME(m_what) printf("MS: %s - %lu\n", m_what, OS::get_singleton()->get_ticks_usec());