Initial commit of 'Data is empty' condition with unit test.
Merge pull request #69 from paranojik/8.x-3.x-dataIsEmpty

#2281081 - Port "Data value is empty (data_is_empty)" condition to 8.x
Move documentation to separate repository.
Merge pull request #70 from fubhy/move-docs

Move documentation to separate repository.
Implementation of the List count comparison condition.
Merge pull request #73 from paranojik/8.x-3.x-listCountIs

Implementation of the List count comparison condition.
Issue #2245039 : Implemented data selector application in RulesState.
Implemented a variable providing test condition, including test case.
Moved provided context variable methods to the interface and fixed unit test.