ucm: add If.Condition.String to match strings and substrings

ucm: add If.Condition.String to match strings and substrings

Case insensitive string match:

If.1 {
	Condition {
		Type String
		String1 "aa"
		String2 "bb"
	}
	True {
		...
	}
}

Substring is present (case sensitive):

If.1 {
	Condition {
		Type String
		Haystack "White Horse"
		Needle "Horse"
	}
	True {
		...
	}
}

Signed-off-by: Jaroslav Kysela <perex@perex.cz>