feat(Timeline): entity queue

[GeopJr]

fix: #126

How does this work?

• If the user's scroll position is near the top: new posts get prepended
• If the user's scroll position is not near the top: new posts get added into an array and when the user scrolls to near the top, they get prepended

[darkdragon-001]

UX wise Tusky on Android is pretty good. One feature I like there in combination with this one here is that you can scroll to the top via click on the current menu item. Also it will automatically start from top after some time / after restarting the app?

[GeopJr]

One feature I like there in combination with this one here is that you can scroll to the top via click on the current menu item

This has been suggested before and the main point was that such pattern is not found in any other GNOME app and might distort its (the view switcher's) expectations in other apps. Like GNOME Software or Music. There's already a scroll to top button anyway. This needs more thought but probably after #228

Also it will automatically start from top after some time / after restarting the app?

(I assume on Tuba, not tusky) This PR shouldn't cause any noticeable changes to users (apart from the scrolled window not moving around when new posts get added) so everything will work like it did before: On start, it starts at the top and at no point does it automatically change the scrolled position without the user triggering it (either through the scroll to top button or manually scrolling to the top)

Just to go a bit deeper on how this PR works in case it wasn't that clear:

(zoom in / open in new tab if the text is not visible)
prior to this PR:

notice how the post you are reading "moved" below when a new post got added

after this PR: