Lua scripts' callbacks aren't always cleared

[YoshiRulz]

<CasualPokePlayer> in particular, callbacks which are set in a callback

BizHawk/src/BizHawk.Client.Common/lua/NamedLuaFunction.cs

Lines 42 to 47 in ed497a7

	// When would a file be null?
	// When a script is loaded with a callback, but no infinite loop so it closes
	// Then that callback proceeds to register more callbacks
	// In these situations, we will generate a thread for this new callback on the fly here
	// Scenarios like this suggest that a thread being managed by a LuaFile is a bad idea,
	// and we should refactor

--TODO write repro script

see also #3036