Support Shift+click to extend message text selection

[dmitry]

When a message has selected text, Shift+click in the same message now extends the selection to the click point, just like in browsers. Previously, Shift+click simply dropped the old selection and started a new one.

Works in regular chats, channels, topics, replies, pinned, and scheduled messages.

How to try: select a few words in a message, then Shift+click somewhere else in the same message - the selection should extend to the click point.

[john-preston]

This is nice! Sadly, we don't track really where the anchor is, so the feel still won't be native (did we select from symbol 20 to symbol 10 or from symbol 10 to symbol 20 and then shift click on symbol 15 - the results should be different, like in browsers, but here we don't save that information). But this a start!