Implement basic chat members completer and fix segfault on ui close

[elros34]

Amount of chat members for one query is limited to 200 by tdlib but that should be enough for usual use case.

[blacksailer]

Hello! thanks for your pull request! I've merged it to 0.7.6. (Noticed your forced pushed update, will check it too)
I want to add support to multiple username mentioning, because it works only on start. Do you have any ideas for implemetation? Because we need to understand what user is typing.

[elros34]

Thanks, I didn't think you will merge it before I remove WIP from title. I basically forced pushed changes to group/channel info page because my previous implementation with one ListView + header worked poorly in some cases.
I wrote some code to handle @username in any place but not everything works correctly yet. I will see if I can fix it.

[elros34]

@username should works now in any place when writing message provided that previous character is not alphanumeric. I didn't check official telegram application sources but looks like it works similar.