Core: Ensure classes match their header filename

Also drop some unused files.

Renamed:
- `core/dvector.h` -> `pool_vector.h`
- `core/io/resource_import.h` -> `resource_importer.h`
- `core/sort.h` -> `sort_array.h`
- `core/string_db.h` -> `string_name.h`

Dropped:
- `core/allocators.h`
- `core/os/shell.h`
- `core/variant_construct_string.cpp`

modules/mono/glue/base_object_glue.cpp

@@ -33,7 +33,7 @@
 #ifdef MONO_GLUE_ENABLED
 
 #include "core/reference.h"
-#include "core/string_db.h"
+#include "core/string_name.h"
 
 #include "../csharp_script.h"
 #include "../mono_gd/gd_mono_internals.h"