libgimpwidgets: new GimpFileChooser widget.

This widget will replace our usage of GtkFileChooserButton, except that
it has a title label and also it contains GUI implementation for the
SAVE and CREATE_FOLDER actions (unlike the GTK button). Moreover this
widget was removed in GTK4. So it is a good idea to start encapsulating
such GTK widget anyway.

I'm also adding a propwidget function to create such a widget bound to a
GimpParamSpecFile property.

New functions:

- gimp_file_chooser_get_action
- gimp_file_chooser_get_file
- gimp_file_chooser_get_label
- gimp_file_chooser_get_label_widget
- gimp_file_chooser_get_title
- gimp_file_chooser_get_type
- gimp_file_chooser_new
- gimp_file_chooser_set_action
- gimp_file_chooser_set_file
- gimp_file_chooser_set_label
- gimp_file_chooser_set_title
- gimp_prop_file_chooser_new

po-libgimp/POTFILES.in

@@ -56,6 +56,7 @@ libgimpwidgets/gimpcolorselect.c
 libgimpwidgets/gimpcolorselection.c
 libgimpwidgets/gimpcontroller.c
 libgimpwidgets/gimpdialog.c
+libgimpwidgets/gimpfilechooser.c
 libgimpwidgets/gimpfileentry.c
 libgimpwidgets/gimphelpui.c
 libgimpwidgets/gimpicons.c