Introduce an entirely new system for handling message preview copy

This reverts earlier changes made to textForEvent as they are no longer needed.

This also implements an entire tree of textForEvent-like behaviour as the previews need to be different, which is easiest done with its own stack.

src/stores/room-list/previews/MessageEventPreview.ts

@@ -0,0 +1,55 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { IPreview } from "./IPreview";
+import { TagID } from "../models";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { _t } from "../../../languageHandler";
+import { getSenderName, isSelf, shouldPrefixMessagesIn } from "./utils";
+import ReplyThread from "../../../components/views/elements/ReplyThread";
+
+export class MessageEventPreview implements IPreview {
+    public getTextFor(event: MatrixEvent, tagId?: TagID): string {
+        let eventContent = event.getContent();
+
+        if (event.isRelation("m.replace")) {
+            // It's an edit, generate the preview on the new text
+            eventContent = event.getContent()['m.new_content'];
+        }
+
+        let body = (eventContent['body'] || '').trim();
+        const msgtype = eventContent['msgtype'];
+        if (!body || !msgtype) return null; // invalid event, no preview
+
+        // XXX: Newer relations have a getRelation() function which is not compatible with replies.
+        const mRelatesTo = event.getWireContent()['m.relates_to'];
+        if (mRelatesTo && mRelatesTo['m.in_reply_to']) {
+            // If this is a reply, get the real reply and use that
+            body = (ReplyThread.stripPlainReply(body) || '').trim();
+            if (!body) return null; // invalid event, no preview
+        }
+
+        if (msgtype === 'm.emote') {
+            return _t("%(senderName)s %(emote)s", {senderName: getSenderName(event), emote: body});
+        }
+
+        if (isSelf(event) || !shouldPrefixMessagesIn(event.getRoomId(), tagId)) {
+            return body;
+        } else {
+            return _t("%(senderName)s: %(message)s", {senderName: getSenderName(event), message: body});
+        }
+    }
+}