Dedicated keybind to activate flying? & hold/toggle option

[muzikbike]

This request is related to #53. In #53, double-pressing the "move forward" key will cause the player to start sprinting, and that ticket raises the issue that this functionality cannot be unlinked from the move forward key and assigned to some other key.

Flying is a similar case - it is activated by double-pressing the key that causes the player to jump. The problem is that it too follows the jump key around, which is undesirable. It'd be expected that by default, the "jump" and "start flying" keys would both be bound to spacebar, and could be rebound to any desired key. Pressing the "start flying" key once, if the jump key isn't bound to the same key as it, would immediately cause the player to start flying.

And for consistency with sneaking and sprinting, there would be an option to change whether this key toggles flying when pressed, or if it should be held down continuously to remain in flying mode.

I would like a dedicated flying key a lot, since my computer lags a lot and sometimes it takes a lot of spacebar spamming to have the game register that I want to fly. The ability to split that key from the spacebar and only have to press it once to initiate fly mode would make things a lot easier.