add saving and full editing control of custom beat signatures when mapping + remove "unsnapped" note warnings when snapped to any non standard yet existent signature

[Miralko]

would be really convenient for multiple sessions of mapping a song with less orthodox time signatures or off-typical beat sound effects, maybe we could get more m̶a̶c̶h̶i̶n̶e̶g̶u̶n̶ ̶p̶s̶y̶s̶t̶y̶l̶e̶ options for signature like decimals, but togglable off for... ...well, pretty much everyone who isn't going to use it
the thing that got me a bit salty over those warnings specifically were some 1/16th lasting microholds (mania if context needed) which had to be placed on 6ths, hence my use of 1/48, getting warnings each