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### Problem description
<!-- Describe the bug that you fixed/feature request that you
implemented, or link to an existing issue describing it -->
Fixed possible bug of `EventManager::unsubscribe`
`std::map` only allows unique key, but the same token can subscribe to
multiple events.
1a2a926b77/lib/libimhex/include/hex/api/event.hpp (L104-L107)
If the previous token has already subscribed to an event, then when
subscribing again, `getTokenStore().insert` will not do anything
(Because its type is `std::map`)
1a2a926b77/lib/libimhex/include/hex/api/event.hpp (L122-L134)
At this point in `unsubscribe`, the `iter` may not be able to find the
correct event and erase it
### Implementation description
<!-- Explain what you did to correct the problem -->
Change `tokenStore` to `std::multimap` instead of `std::map`, which
cannot unsubscribe multiple events correctly
### Screenshots
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make before/after sceenshots -->
### Additional things
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