Merge AbstractCSSEngine into CSSEngineImpl

The CSS core engine had two stacked abstract classes with no concrete
core implementation: AbstractCSSEngine (1,113 LOC) and a tiny
CSSEngineImpl (95 LOC) that only added parser-factory wiring,
property-handler registration helpers, and the boolean value
converter. Inline the small subclass into the base, keep the name
CSSEngineImpl since that is the type SWT engine subclasses and tests
already reference, and make makeCSSParser concrete (it was abstract
solely so CSSEngineImpl could supply its single implementation).
The class remains abstract because the CSSEngine.reapply() method has
no sensible default in the core bundle.

Update ParserTestUtil's parseCssWithoutImports cast and rename
AbstractCSSEngineTest to CSSEngineImplTest in the same pass to keep
test class names aligned with production class names. The test's
anonymous subclass no longer needs to override makeCSSParser since it
is now concrete on the base.

All bundles internal (x-friends only); no API surface changed.

Contributes to #3980

Author: vogella