Fix indentation/fontification of Java enum with "implements"/generic.

Fix indentation/fontification of Java enum with "implements"/generic.

* progmodes/cc-engine.el (c-backward-over-enum-header): Extracted from
the three other places and enhanced to handle generics.
(c-inside-bracelist-p): Uses new function above.
* progmodes/cc-fonts.el (c-font-lock-declarations): Uses new function
above.
(c-font-lock-enum-tail): Uses new function above.