* src/stringutils.{cc,h} (expand_pathname): Operate directly in
filename encoding since all callers need to convert one way or
the other anyway.  Change type of parameter and return value to
std::string accordingly.
(shorten_pathname): ditto.

* src/mainwindow.cc (MainWindow::MainWindow): Reverse order of
the calls to filename_to_utf8_fallback() and shorten_pathname().
(MainWindow::on_select_folder): ditto
(MainWindow::on_find_files): ditto.

* src/mainwindow.{cc,h} (MainWindow::set_title_filename): Change
parameter type to std::string and call filename_to_utf8_fallback().
Also remove the default argument from the declaration, and get rid
of the logic that handles the empty string.
(MainWindow::MainWindow): Call window_->set_title() directly in
order to set the default title, instead of passing an empty string
to set_title_filename().
(MainWindow::on_filetree_switch_buffer): ditto.