Scheduled Transaction formula support string literal parameters.

[Yaocl]

Hi

>> Moreover, you need the *date* in there, because you really want to be
>> able to call xaccAccountGetBalanceAsOfDate() in the SX, because the SX
>> is run at a particular date that may be in the past.
> I agree, I must get scheduled date from somewhere.
I tried to add implicit var of *date*, Although var_store support
numeric and string types but the make_predefined_vars_helper only
handle predefined vars of numeric type.

Because it is hard to modify so many aspect. I decide to keep my
approach, hard coding the account (name or guid) in my scm file, get
date based on the `i`, this will fit my needs.