Swap the directory tree filtering to method-based

This lets developers subclass DirectoryTree and implement their own filter.

src/textual/widgets/_directory_tree.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import os
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Callable, ClassVar
+from typing import ClassVar, Iterable
 
 from rich.style import Style
 from rich.text import Text, TextType
@@ -90,11 +90,9 @@ class DirectoryTree(Tree[DirEntry]):
         id: str | None = None,
         classes: str | None = None,
         disabled: bool = False,
-        path_filter: Callable[[Path], bool] | None = None,
     ) -> None:
         str_path = os.fspath(path)
         self.path = str_path
-        self._path_filter = path_filter or (lambda _: True)
         super().__init__(
             str_path,
             data=DirEntry(str_path, True),
@@ -152,12 +150,23 @@ class DirectoryTree(Tree[DirEntry]):
         text = Text.assemble(prefix, node_label)
         return text
 
+    def filter_paths(self, paths: Iterable[Path]) -> Iterable[Path]:
+        """Filter the paths before adding them to the tree.
+
+        Args:
+            paths: The paths to be filtered.
+
+        Returns:
+            The filtered paths.
+        """
+        return paths
+
     def load_directory(self, node: TreeNode[DirEntry]) -> None:
         assert node.data is not None
         dir_path = Path(node.data.path)
         node.data.loaded = True
         directory = sorted(
-            [entry for entry in dir_path.iterdir() if self._path_filter(entry)],
+            self.filter_paths(dir_path.iterdir()),
             key=lambda path: (not path.is_dir(), path.name.lower()),
         )
         for path in directory: