This separates the "ground truth" store of hotkeys from what is shown
in the dialog. This will allow us to filter the displayed hotkeys
while keeping the same underlying data structures.
Now, the UI data items interact with an intermediate set of data, which
represents the "original" hotkey data, and the "changed" data. The
ultimate aim here is to allow UI elements to come and go, but the
hotkeys that are "in-edit" are preserved.
This also allows us to abstract some bookkeeping complexity
out of the WIDGET_HOTKEY_LIST class into a separate non-GUI
class.
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