When opening a file dialog to set the location of a custom path, use the
corresponding user path as the starting location instead of the current
custom path.
When no custom path was set the dialog would be opened with a blank path
which causes Windows (not sure about other platforms) to open the dialog
at the same location where the last dialog was closed, or at the current
working directory if no previous dialog had been opened.
If a nonempty custom path has been set then it has also set the
corresponding user path, so the behavior in that case is unchanged.