GtkFileChooserEntry: regenerate the completions if the dir_part changes

GtkFileChooserEntry: regenerate the completions if the dir_part changes

Consider this bug:

1. Open a file chooser; switch it to $HOME

2. Start typing "~/Dow" with some file that *does* exist in your $HOME

3. Delete the inline-completion selection (e.g. the "nloads" after "~/Down").

4. While you are at "~/Dow_" hit Tab.  No completion will occur.

This happens because of the following.

Say the GtkFileChooserEntry is in the process of loading $HOME,
because _set_base_folder() was called.  If the entry contains no text,
then the FULL_PATH_COLUMN of the file system model will be set to
unprefixed filenames from $HOME, like

  .ssh/
  Documents/
  Downloads/
  somefile.txt

Later we avoid reloading the folder if g_file_equal(old_folder, new_folder).

However, the FULL_PATH_COLUMN gets populated in completion_store_set()
out of the actual filenames that GIO returned, plus the chooser_entry->dir_part.

If the user starts typing "~/Dow" then dir_part changes to "~/", *but*
the folder won't be reloaded since it is also $HOME.  However, the completion
machinery assumes that FULL_PATH_COLUMN will contain prefixed entries like

  ~/.ssh/
  ~/Documents/
  ~/Downloads/
  ~/somefile.txt

So, we add an invariant that chooser_entry->dir_part and
chooser_entry->current_folder_file must change at the same time, and
must not get out of sync:  If any of them changes, then the
completions are regenerated.