[Vanilla Bugfix]: corrected create_hud_item logic for duplicate hud section (closes #1359)

Created by: yohjimane

Fixes issue 1359

The root issue is this function:

attachable_hud_item* player_hud::create_hud_item(const shared_str& sect)
{
    auto& item = m_pool[sect]; // each key in m_pool is set to hud_sect. These are not unique.

    if (!item)  // If equipped item 1 hud_sect == equipped item 2 hud_sect, the line below is not called, causing the bug.
        item = xr_new<attachable_hud_item>(this, sect, m_model);

    return item;
}

The fix:

1. Add instance id m_inst_id to CHudItem, which gets incremented any time a new item gets loaded.
2. Add m_unique_hud_sect to CHudItem, which = hud_sect + m_inst_id
3. Refactor create_hud_item to take CHudItem as parameter & key m_pool by m_unique_hud_sect to avoid duplicate item bug.