Fix file chooser entry not updating

Save file name while it is typed in the text entry.

Add placeholder text for Lensfun database directory to indicate
automatic detection if left blank.

rtgui/guiutils.h

@@ -463,6 +463,9 @@ class MyFileChooserEntry : public Gtk::Box, public MyFileChooserWidget
 public:
     explicit MyFileChooserEntry(const Glib::ustring &title, Gtk::FileChooserAction action = Gtk::FILE_CHOOSER_ACTION_OPEN);
 
+    Glib::ustring get_placeholder_text() const;
+    void set_placeholder_text(const Glib::ustring &text);
+
 protected:
     void on_filename_set() override;