import-dialog: Add a toast overlay

This toast overlay will be used to display error messages
in import operations, namely, files that could not be
imported in a drag and drop operation

src/gui/importer/gcal-import-dialog.h

@@ -23,6 +23,7 @@
 #include <adwaita.h>
 
 #include "gcal-context.h"
+#include "gcal-import-file-row.h"
 
 G_BEGIN_DECLS
 
@@ -33,4 +34,10 @@ GtkWidget*           gcal_import_dialog_new_for_files            (GcalContext
                                                                   GFile             **files,
                                                                   gint                n_files);
 
+GtkWidget*           gcal_import_dialog_new_for_file_list        (GcalContext        *context,
+                                                                  GList              *file_list);
+
+void                 gcal_import_dialog_add_toast                (GcalImportDialog   *self,
+                                                                  AdwToast           *toast);
+
 G_END_DECLS