Hide goods with an invalid category from goods lists

[BrknRobot]

This is so that goods can be used to selectively enable PMs, without cluttering up the goods lists.

For example, the following could be used to select PMs based on whether a state is coastal, without any additional goods showing in the UI

# common/goods/hidden_goods.txt
coastal_state = {
	texture = "gfx/interface/icons/goods_icons/clippers.dds"
	cost = 1
	category = hidden
	local = yes
}

# common/static_modifiers/99_code_static_modifiers_overwrite.txt
coastal_state = {
	icon = gfx/interface/icons/timed_modifier_icons/modifier_gear_positive.dds
	state_infrastructure_from_population_max_add = 10
	state_infrastructure_from_population_mult = 0.05
	state_sell_orders_coastal_state_add = 1
}

# common/production_methods/enabled_by_coast.txt
inland_pm = {
	replacement_if_valid = coastal_pm
	...
}

coastal_pm = {
	required_input_goods = coastal_state
	...
}