Enhancement: ability to set a static scroll speed across all charts

[platonictf]

currently, scroll speed is set arbitrarily by whoever charted the song you are currently playing. this leads to various playability issues, such as scroll speeds varying wildly between songs/difficulties, which can throw off players not expecting it as well as newbies who cant play with a fast scroll speed.
it also makes most songs, particularly ones with slow scroll speed generally uncomfortable to read for experienced players who are used to reading much faster scroll speeds, usually resulting in them hitting notes very early and/or misreading patterns entirely that they otherwise wouldnt have.

allowing the player to set a single scroll speed for every song would solve all these issues, as newbies won't be caught off guard with a scroll speed that is way too fast, as it now remains the same across all songs, and experienced players can use whatever speed is most comfortable for them to read.