Make inherit_ryukyu be regulated mainly by the allow and not the potential

[rogerburks]

References #152

As a request from someone else who played as Japan, I intend to move much of the potential to the allow, while keeping the value of the potential as a gatekeeper for visibility. The point of this is to make the requirements for annexing Ryukyu more visible, so that the player can better plan to do this effectively. Player protection remains.

Specifically, I mean this text:

ccHFM/ccHFM/decisions/Japan.txt

Lines 1247 to 1261 in 47becfb

	OR = {
	tag = QNG
	AND = {
	tag = JAP
	has_country_modifier = fukoku_kyohei
	OR = {
	has_country_flag = taiwan_line_of_advantage
	revolution_n_counterrevolution = 1
	}
	}
	AND = {
	tag = TKG
	has_country_modifier = fukoku_kyohei
	}
	}

The potential has a section added to ensure that only Qing, Japan, and Tokugawa Japan can see it.