Fix crash when adding an external editor

On some platforms, the app chooser dialog causes a crash after selecting
an application. The application information returned by the dialog may
also trigger crashes when accessed. See
https://gitlab.gnome.org/GNOME/glib/-/issues/1104 and
https://gitlab.gnome.org/GNOME/glibmm/-/issues/94. This commit overrides
gtkmm's app chooser dialog to work around these bugs.

rtgui/rtappchooserdialog.h

@@ -0,0 +1,39 @@
+/*
+ *  This file is part of RawTherapee.
+ *
+ *  Copyright (c) 2021 Lawrence Lee <billee@ucdavis.edu>
+ *
+ *  RawTherapee is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  RawTherapee is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with RawTherapee.  If not, see <https://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <giomm/appinfo.h>
+#include <glibmm/refptr.h>
+#include <glibmm/ustring.h>
+#include <gtkmm/appchooserdialog.h>
+
+/**
+ * Custom version of gtkmm's Gtk::AppChooserDialog to work around crashes
+ * (https://gitlab.gnome.org/GNOME/glib/-/issues/1104 and
+ * https://gitlab.gnome.org/GNOME/glibmm/-/issues/94).
+ */
+class RTAppChooserDialog : public Gtk::AppChooserDialog
+{
+public:
+    RTAppChooserDialog(const Glib::ustring &content_type);
+    ~RTAppChooserDialog();
+
+    Glib::RefPtr<Gio::AppInfo> get_app_info();
+    Glib::RefPtr<const Gio::AppInfo> get_app_info() const;
+};