Fixes some errors with HeadGetter

[BONNe]

There were multiple issues in HearGetter.

1. If someone 2 GUI's requested player heads at the same time, then one of them could easily throw ConcurrentModificationException, as it used Map structure, where it is not necessary.
2. If the creator wanted 2 different icons with the same player head, it was not possible, as implementation allowed only one player head with the same name, due to using a hashmap.
3. There were unnecessary code that overwrites all PanelItems with the same name if they were requested by Head Getter. This also prevented to use the same name for PanelItem but with different player heads.

Also, I added Pair#getKey and Pair#getValue as I like to use getters more than variables directly.

[BONNe]

Level addon definitely will work. You are creating different PanelItems for each button.

The only way how it can fail to update GUI is if someone creates a single instance of PanelItem, and uses it multiple times in the same GUI.

I am not sure how useful to support that feature, but it is not hard. Just need to find all usages of that PanelItem instance in the Items entrySet, and update each item in Inventory.

But I do not think this is a useful feature to support, as I cannot find any good use case for it.