file chooser entry: Capture Escape and emit :hide-entry

file chooser entry: Capture Escape and emit :hide-entry

Make the file chooser entry optionally capture Escape
and emit a signal. Make the file chooser widget hide the
entry on that signal and go back to the path bar.
This gives us a two-level undo:
location entry -> path bar -> dialog close.
When the location entry is permanently displayed in the
header for save mode, we still let the first Escape close
the dialog.