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`

core/math/geometry.h

@@ -31,12 +31,12 @@
 #ifndef GEOMETRY_H
 #define GEOMETRY_H
 
-#include "core/dvector.h"
 #include "core/math/face3.h"
 #include "core/math/rect2.h"
 #include "core/math/triangulate.h"
 #include "core/math/vector3.h"
 #include "core/object.h"
+#include "core/pool_vector.h"
 #include "core/print_string.h"
 #include "core/vector.h"