Floating bullet fix

This change renames round_in_chamber to active_round, and then only removes bullets from the magazine when going from the RoundState.LOADING state to RoundState.READY. If a magazine is removed when in the LOADING state, the active_round is destroyed. This fixes the floating round bug mentioned by myself (https://github.com/David20321/7DFPS/issues/1) and by mossarelli (https://github.com/David20321/7DFPS/issues/5).