ph-css 6.0.0

• Extracted base class AbstractHasTopLevelRules for type-safe access of top-level rules (#39)
• Updated to ph-commons 9.0.0
• Extended API to more easily access the CSS declaration expression string (#43)
• Data URI scheme is treated case-insensitive (#44)
• Extended API for easier CSS property comparison (#45)

Dependencies required:

• ph-commons 9.0.0