Add “game” mode.

[kpreid]

There should be a “game” mode, which restricts player actions to those allowing a specific game to be played, as determined by the blockset and possibly other game-parameter objects.

Straightforward restrictions:

• Do not allow editing block types.
• Do not allow editing the world globally, once that feature is present. (Perhaps that can be made into carriable tools which the player has no inventory of?)
• Do not allow flying or noclip (or at least take flying out of a main keyboard control).
• Do not allow teleportation (i.e. remember player character position on quit and reload).
• Do not display player coordinates and mouseover info.

Game-rule-dependent restrictions:

• Restrict availability of block types to place.
• Restrict deletion of blocks. (Define in block circuits? Would be problematic for high-population blocks to be circuited, unless we use a more fine-grained "does this need evaluation" scheme.)

The above items would suffice to close this issue.

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Notes of further, optional, elements which are not universally necessary, but probably needed for a lot of game designs:

Game features:

• Character inventory/stats.
• Character death.

Non-mode-requiring game parameters:

• Gravity magnitude and direction.
• Character size, movement speed, jump height, air-steering capability, slope walking capability.
• Reach of cursor.