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/csg/csg.cpp

@@ -32,7 +32,7 @@
 #include "core/math/face3.h"
 #include "core/math/geometry.h"
 #include "core/os/os.h"
-#include "core/sort.h"
+#include "core/sort_array.h"
 #include "thirdparty/misc/triangulator.h"
 
 void CSGBrush::clear() {