The menu entry "Move to _trash" as well as the toolbar tooltip "Trash Message" refer to the same program action. Thus they should consistently be called "Move to _trash"

```
$ git grep 'Trash Message\|Move to _trash'
mainwindow.c:   {"Message/Trash",                            NULL, N_("Move to _trash"), "<control>D", NULL, G_CALLBACK(delete_trash_cb) },
toolbar.c:      { "A_TRASH",            N_("Trash Message")                        },
toolbar.c:                      TOOLBAR_ITEM(item,icon_wid,toolbar_item->text,_("Trash Message"));
```