Feat/improve player controls

[piellardj]

This PR improves the player controls:

• pressing simultaneously left and right mouse button makes the player go forward
• added distinction between "turn left/right" and "strafe left/right"
• moving the player while pressing right mouse button makes the player follow the camera position
• moving the player while pressing left mouse button fixes the camera independently from the player movement
• moving the player while not pressing left/right mouse button makes the camera follow the player movement
• add support for first-person view

Also, this PR fixes two bugs:

• losing window focus while moving the player made the player run indefinitely without user input, because we did not register the window.keyup event
• css: top/bottom panels prevented mouse interaction which was confusing

[Sceat]

Thanks, testing the feature I like some, and not some others:

• ✅ pressing simultaneously left and right mouse button makes the player go forward
  • can't fully test right now with only MBP pad, but it's a "must have" feature 👍🏻
• 🚫 moving the player while pressing right mouse button makes the player follow the camera position
  • Initially left button, doesn't change much, but it can interfere with custom right click drop down used to interact with other players (could use left click for that tho)
• 🚫 moving the player while pressing left mouse button fixes the camera independently of the player movement
  • the trailing effect can already be used instinctively by moving in a direction and using original left click camera, I find it confusing.
• 🚫 moving the player while not pressing left/right mouse button makes the camera follow the player movement
  • this looks too much like a retro game, not really enjoyable
• ✅ add support for first-person view
  • this is cool!

[piellardj]

I removed the features you didn't like.