ContextMenu: Reformat & refactor

Started as adding qualified property access (basically prepending "root."
to everything), but quickly has turned into something a bit larger. All
visibility and enabled state conditions were factored out into top-level
helper predicate functions which not only reduced code duplication but
also improved readability. Added type hints to spellcheck-related
properties (even though the feature is broken as of commit
e81de1ed).