Adrián Pérez de Castro authored Mar 12, 2017 and Michael Catanzaro committed Mar 12, 2017

This fixes bug #779922, making Epiphany a better citizen in the world of
sanboxed applications, and potentially when running under non-GNOME DEs
as well.

The ephy_file_chooser_new() function is turned into an utility function
which instantiates a GtkFileChooserNative under the hood, returning it
as a GtkFileChooser* to keep most of the rest of the Epiphany code
unchanged, saving for:

- Calls are changed to gtk_native_dialog_* where appropriate.

- The preview widget is gone, as it is not supported by native dialogs.

- All the calls to ephy_file_chooser_new() pass in a valid parent
  GtkWindow, so the code that handled NULL parents is gone.