General override/implement member/function/variable operation

[themkat]

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A override-method/member operation like many Java plugins has (e.g, lsp-java for Emacs). Not just abstract/unimplemented ones, but making it possible to override methods/members in general. Some interfaces have default implementations now, so these will probably not show up as abstract.

A lot of the Implement Abstract Members Quick Fix code can probably be reused with some minor tweaks.