Let everybody pause; add `gadget:AllowPause(playerID, paused)`

Bill of materials to be done before starting this ticket:
* #1372

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Right now the engine hardcodes it so that players, and only players, can pause (i.e. specs can't pause, and you can't block a player from pausing). The engine should expose an interface for games to decide that instead:
* a basecontent gadget, or perhaps a default in case the game doesn't return an explicit true/false, that keeps current behaviour blocks it for spectators.
* most games would probably want to allow the host/boss of a room to pause even if he's a spectator.
* you could make it so everybody has 3 pauses.
* you could make it so that in a tournament, the players can't pause, only the judge.
* this would probably have to be a synced interface so replay watchers would be exempt and always able to pause.
* this would probably have to be a synced interface so you could even do something silly like make people pay metal for pausing.

A synced `Spring.SetPause(bool)` would be good too.

Review prerequisite issue #1372 and the proposed gadget:AllowPause(playerID, paused) interface first. Clarify whether the synced Spring.SetPause(bool) API is also required; done means games can control who pauses while replay watchers remain exempt and current behavior has a default.

Written by the indexing model from the issue text.