Rework default config binding replacement

This reworks the criteria necessary for default bindings to be
overwritten by custom user bindings. It should provide a better
heuristic which accounts for the possibility that notmodes are added
which prevent a conflict between the bindings, so the default isn't
unnecessarily removed.

It is still possible to define a new binding that intersects a default
binding without the default getting removed, if the user explicitly
specifies a mode that doesn't include the default binding's mode. This
is based on the assumption that users explicitly specifying a new mode
are trying to make a mode-specific addition and are capable of removing
the default when desired.

This helps with old modes still being present in user's config files,
since a lot of new config options just have the additional ~Vi mode set.

Fixes #3476.

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