fix thumbnail prevew animation regression

[theScrabi]

Dear @Stypox when you simplified the ThumbnailPreview you introduced a regression :P. The getHeight() function as well as the Box inside the AnimatedVisibility is necesary to prevent glitches to appear when fading in or out. This is because compose does not seem to be able to calculate the size and position of the thumbnail correctly during animation. For this reason we just animate the whole strip where the thumbnail can move on.

With glitches:
with_glitch.webm

Glitches fixed:
glitch_fix.webm

[theScrabi]

This will take a while to fix,
The chapter title preview destorys the position calculation.

[Stypox]

Mmmh I remember noticing the same issue both with and without #20 but maybe I remember wrongly. Does the problem happen also on an emulator or only in previews?

[theScrabi]

It happens in preview, the emulator and on device. The title preview isn't the issue though. This PR already delivers a fix for the issue. However additionally I noticed that, #18, made the preview thumbnail position calculation be wrong now. In order to apply my fix right now, we'd have to roll back the chapter thumbnail preview.
So I am going to work on a correct chapter preview text positioning as well :) This might take a while though.

[theScrabi]

Ok so I learned today that SubcomposeLayout can fix all of my issues. Now the layout works well. No glitches, and the chapter text is independently placed.

Please note that I renamed chapter preview text to thumbnail preview text. This way also things like the playback position could be encoded into the text.