My implementation of extra buttons requested by issue 2743

[Lainadux]

I implemented the feature requested in #2743 and merged the "player fit to win button" with the newly added button into a submenu called "Extra buttons inside the player."
This might be a bit off-topic, but during debugging, I noticed that the "player fit to win button" toggle only works once (i.e., if I toggle this option multiple times on the same page, it works as expected the first time—creating and hiding the corresponding button in the bottom right corner of the video—but subsequent toggles fail to work properly). I suspect this is related to an issue encountered during development: buttons created using the createPlayerButton API do not display correctly unless it-player-button class is removed.

[ImprovedTube]

our structure is redundant (considering this could be at any position and also is at shortcuts)
(Feature/function: Seek, Duration: X; Shortcut: Y; Button (Position: Controls, right) )