cxxll::java_class: Switch to PIMPL idiom
cxxll::java_class: Extract field and method definitions

Ths adds member classes cxxll::java_class::accessible,
cxxll::java_class::field, cxxll::java_class::method and member functions
cxxll::java_class::fields(), cxxll::java_class::methods().
SQL schema: Add tables symboldb.java_field_definition, symboldb.java_method_definition
database::add_java_class(): Extract field and method definitions