Major feature update to YouTube Full Theater#1

Hello Isaiah, Thank you for creating the Full Theater userstyle, it is very useful. I'm happy that it works much better than what I used before. I took it further by addressing a few edge cases and adding some transparency. I wonder if you'd like to integrate some of these changes? Each commit is one atomic change. Otherwise I can make a fork, but first I wanted to ask your intentions. Per commit, the issues I've fixed, with reproduction: 1. Code tidying. 2. Scroll down half screen, press '/' or click in searchbox -> search suggestions appear mid-screen. 3. In theater mode go fullscreen, move the mouse to about 20px from the top in the center, click -> search suggestions appear (searchbox, header is not hidden). 4. Click in searchbox, move mouse out of header -> header is hidden, but searchbox is active. 5. Refactoring. 6. In theater mode the header covers the info button if the video aspect ratio is as tall as the screen. 7. Comments. 8. The goal was to tune the animations to be as unnoticeable as possible, to avoid distraction. 9. Same. 10. Reorganizing. 11. Youtube's design is quite haphazard in this case. 12. Goal: to be seamless. 13. To have more useful screen estate. 14. The title and the top-right buttons are visible when and only when the mouse is above that area.