Add "lock BPM" function to Double time mod

[emilow12]

I think the title is quite self-explanatory.
An option to lock the double time mod to certain bpm ( either of current map or writeable ), not sure how to handle if it exceeds 2x multiplier though ( maybe cap it to whatever 2x is, in such case? ) also not sure if it goes below 1x, automatically change mod to half time then?

I think it would be really cool addition to the game and make many folks happy.

Inspired by reddit post and my own thought from the past.

https://www.reddit.com/r/osugame/s/aw5lI4YVYj

[bdach]

#28138
#22616
etc