Fixes and a fps camera

I created a first person camera that can be activated through the artist menu. It's possible to move with WASD keys and rotate with Q and E. This only works if no keyboard configuration uses that keys (like with I and J to fly with a kart). Pressing ESC returns to the normal camera.

Other changes:
* Modifies the multiplayer kart selection so no player is autoselected at the beginning
* Deactivate checkboxes in the custom video settings screen to make possible settings more obvious
* Some minor changes
This was already there so of course it's not your fault, but I just realised this code is far from ideal, this text is clearly not meant to be displayed in-game, yet it will be extracted for translators. So if this text is not meant to be shown, while you're at it, it should only be text="". If we want a hint that something will be filled at runtime, this hint should be done with a <!-- xml comment -->, which will not bother translators needlessly