This change allows to pass optional freeing and copy callbacks to
functions setting the `func_argN` member of the `uiBut` and `uiBlock`
structs. These callbacks are used to free or duplicate the data stored
in these void pointers.
Defaults are set respectively to `MEM_freeN` and `MEM_dupallocN`, to
match previous hard-coded behavior.
Utils template functions are added to easily generate callbacks to
types that are managed with the `MEM_new`/`MEM_delete` C++-style
allocation, as long as they provide a copy constructor.
Note that this change should be considered as a temporary, transitional
solution. A more robust and future-proof solution is to move towards
more usage of `std::function` for these callbacks, which can then own
and manage their custom data themselves.
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