HotkeySystem set_config conflict merges badly

[AlexKnauth]

If a HotkeySystem starts out with the default bindings, and then set_config is used to replace those with new bindings, it behaves badly when one of those new bindings conflicts with one of the old bindings.

Old bindings:

Split	Numpad 1
Reset	Numpad 3
Undo	Numpad 8
Skip	Numpad 2
Pause	Numpad 5

PrevCmp	Numpad 4
NextCmp	Numpad 6

New bindings:

Split	Numpad 0
Reset	Numpad 7
Undo	Numpad 4
Skip	Numpad 1
Pause	Pause Break

When it goes to set Undo to Numpad 4, it still sets that in the config, but then signals a failure that leaves the old bindings from Skip onward in place to result in

Badly-merged bindings:

Split	Numpad 0
Reset	Numpad 7
Undo	Numpad 4
Skip	Numpad 2
Pause	Numpad 5

PrevCmp	Numpad 4
NextCmp	Numpad 6

With a conflict between Undo and PrevCmp staying in the config because it didn't go on to set PrevCmp to None.

My desired behavior would be to un-register the conflicting PrevCmp binding before it registers the new Undo binding as Numpad 4.