It seems the best way to do this is to add a `handler` callback to the modifier `PanelType`s.
It's actually a pretty elegant way to handle this customization in my opinion.
I've added so far:
* `X` and `Delete` for removing the modifier
* `D` for duplicating a modifier. Apparently this can be a common operation for things like rigging.
It also adds little shortcut strings in the menu (I had to add a new flag to the layout code for that).
I'd like to get feedback on the shortcuts (whether there should be more, fewer, different, etc..), then do technical review after.
Other related improvements: D8037, D8040, D8042