Like GtkFileChooser does; GtkOpenWith is a generic interface, which is
now implemented by both GtkOpenWithDialog and GtkOpenWithWidget (and in
the future also by GtkOpenWithComboBox).
We have three modes of showing applications now:
- RECOMMENDED mode: shows only the recommended applications
- ALL mode: shows all applications, uncategorized
- HEADINGS: shows headings for recommended and other applications
(default).
- _set/get_show_other_applications() is a way to only display
recommended applications in the dialog
- _set/get_show_set_as_default_button() is a way to control the
"Remember this application" checkbox
We also get rid of he "application-selected" signal, in favour of a
_get_selected_application() API inspired by GtkFileChooser.