Extend codegen's FunctionDescription with isAbstract/isFinal/isOverriding

Summary:
Enables e.g. file templates snippets to access these properties
(kdevelop's file_templates/common/class_method_declaration_cpp.txt does
this already accidently)

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7099