Adds beginning of paths management

The paths file will be used for everything related to the paths to
synchronise.
Adds the user_select_files functions that allows the user to select
paths

src/unisync/paths.py

@@ -0,0 +1,40 @@
+# Copyright (C) 2025 Paul Retourné
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+import subprocess
+
+def user_select_files(local_dir:str, choice_timeout:int=120) -> list[str]:
+    """
+    Make the user select files in the top directory.
+    Currently uses nnn for the selection,
+    the goal is to replace it in order to avoid using external programs.
+    Returns the list of paths selected.
+    """
+    command = [
+            "/usr/bin/nnn",
+            "-H",
+            "-p", "-",
+            local_dir
+            ]
+    nnn_process = subprocess.Popen(command, stdout=subprocess.PIPE)
+    try:
+        ret_code = nnn_process.wait(timeout=choice_timeout)
+    except subprocess.TimeoutExpired as e:
+        print("Choice timeout expired", file=sys.stderr)
+        raise e
+
+    if ret_code != 0:
+        print("File selection failed", file=sys.stderr)
+        raise subprocess.CalledProcessError("File selection failed")
+
+    paths_list:list[str] = []
+    while (next_path := nnn_process.stdout.readline()) != b'':
+        next_path = next_path.decode().strip()
+        # Make the path relative to the top directory
+        next_path = next_path[len(local_dir):].lstrip("/")
+        paths_list.append(next_path)
+
+    return paths_list
+
+
+