Support timeline position markers

[noiob]

Mastodon 3.0 supports marking the last toot/notification that was shown in a timeline. It'd be cool if Tusky could use that info to supress push notifications I've already seen in the webapp
mastodon/mastodon#11762

[fbatschi]

It would be great to use the position marker to actually sync your position in the timeline, so that you are in sync with what you have already read on other clients. Maybe as an automatic Feature or by pressing a certain button.