Expose string->bool conversion as function

Expose string->bool conversion as function

There is code in several places in extensions which converts
setting or parameter string (such as "true", "yes", "false", "no")
to boolean. Since we already have the code that does in global
functions in wfStringToBool(), it makes sense to expose this code
and reuse it.

Change-Id: I88d98b012ff4bf14fd64a05a9135a6e75cf2d4e7