Customize smart rewind & playback speed display mode

[KingThomas01]

First off, congrats on releasing 3.0! But after using the new interface, I'm noticing a couple things that I think might be nice to update/add.

• More customization over the tabs (in plappa, you can customize the tabs at the bottom of the screen such as series, books, author, collections, etc)
• Allow for customization on the Smart Rewind (I struggle to remember what was going on after taking a break from an audiobook, and 10 seconds is not always enough for me to remember everything so I have to manually rewind when I start again)
• Allow for the option to choose between % and decimal system for playback speed label (and maybe even add a pop out like the sleep timer to quickly choose a specific speed instead of having to tap the speed several times to get which speed you want)
• Lastly, I think the home page could definitely be utilized more effectively and not have so much empty space. Audible's "listen" section is a great example that utilizes the whole screen where there isn't blank space on the sides for the categories like "continue listening", "discover", etc. Here's an example

Thanks for any consideration of the suggestions listed!

[rasmuslos]

I will use https://github.com/rasmuslos/ShelfPlayer/issues/341 to track Home Screen customization. TabBar customization will release in 3.1.0.