Improve room setting

* Port away from OverlaySheet
* Use Kirigami.CategorizedSettings
* Add join rules (read only for now)

src/joinrulesevent.cpp

@@ -0,0 +1,16 @@
+// SPDX-FileCopyrightText: 2019 Kitsune Ral <Kitsune-Ral@users.sf.net>
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include "joinrulesevent.h"
+
+using namespace Quotient;
+
+QString JoinRulesEvent::joinRule() const
+{
+    return fromJson<QString>(contentJson()["join_rule"_ls]);
+}
+
+QJsonArray JoinRulesEvent::allow() const
+{
+    return contentJson()["allow"_ls].toArray();
+}