Ability to enforce osu!mania key count

[aryoadhi]

Ignoring the ability to choose autoconvert (because mostly osu!mania players want to play maps specific to that mode), is there any way to enforce key count? For example, consider this situation:

Room: Force players to choose 4K Host: Chooses 5K map osu-ahr: Checks the map's keycount, compare with the rule. If not same: reverts choice to last played map, warn user for choosing wrong keycount

This can be done just like how MapChecker do, if violated for certain amount, host will be skipped.

[Meowhal]

It may be possible to check the number of keys on the map. This could be accomplished by adding a check item to the map checker. I've added it to TODO List.