add basic UI indication of task scrub mode being active

Changes task scrub Y threshold to be a bit bigger to be nicer and more consistent to use on smaller screens. This should probably go in no matter what we decide on the later part.

Also added UI indication, that one is a bit more "maybe yes, maybe no":

In the long run I think it would be nice to have a list of window icons, but I don't think I'll be able to get to that in the next few weeks so this could be interesting as an interim solution for 6.1 (and it might also be a part of the "proper" long term UI as well?)

I have two ideas for indicator design:

Version 1

Here the idea is that when task scrub mode is activated the arrows are left and right of the finger and point in which direction you can still swipe (and get greyed out when you reach the end of the list. the arrow direction points towards how the finger would need to move)

Version 2

Probably not going with that one, see comment further down, example screenshot still visible in collapsible section:

Expand

Here the UI hint ("Swipe" and the arrows) is biased a bit upwards instead of in the center because when shown the user still has their finger on the screen. This has more info (it says "swipe") and is more easily visible, no matter how far left/right the finger is, but it also feels more abstract and weird looking?

I prefer Version 1 because it feels more intuitive, cleaner and looks more likely that we can have it alongside a window icon list, but maybe we don't want either and implement a window icon list later on and just leave scrub mode a bit barren in 6.1? Would be happy to hear feedback

This MR addresses some points of #367