Corrected PHP type for "decimal" mapping type

"Basic Mapping" documentation says:
"decimal: Type that maps a SQL DECIMAL to a PHP string."
Updated EntityGeneratorTest::testEntityTypeAlias
Updated EntityGeneratorTest::testEntityTypeAlias
Merge branch 'DDC-2587' into 2.4
Skip not mapped public properties in SchemaValidator
Merge branch 'DDC-2577' into 2.4