Remove, edit and query player map pins

[Hellrespawn]

There is currently no way to remove or edit map pins programmatically. LuaPlayer exposes add_pin, but it returns nothing so there is no way to interact with them after creation like map tags.

As a minimal API, returning some sort of internal ID and adding remove_pin(id) to LuaPlayer would be nice. Ideally, we would also be able to query a player's pins and have something similar to LuaCustomChartTag.

[Rseding91]

Ok, for the next experimental release I did the following:

- Added LuaPin.
- Changed LuaPlayer::add_pin() to return LuaPin.
- Added LuaPlayer::get_pins() and clear_pins().