There is no API method to remove a file or subdirectory from a MutableTree
besides directly manipulating the GHashTable returned by _get_files or
_get_subdirs. This isn't possible from an introspection binding that transforms
the returned GHashTable, and may also leave the tree checksum in an invalid
state. Introduce a new method so that removing files or subdirectories is
safe, and possible from bindings.
Closes: #1724
Approved by: jlebon