utils/types: Add file_path type

This can be used to allow extension parameters that are paths to use
'~' to refer to the home directory.

wlauto/utils/types.py

@@ -83,6 +83,9 @@ def numeric(value):
             return ivalue
     return fvalue
 
+def file_path(value):
+    """Handles expansion of paths containing '~'"""
+    return os.path.expanduser(value)
 
 def list_of_strs(value):
     """