Rework sliders in osu! flashlight difficulty calculation

[molneya]

This changes some things with sliders:

• Considers heads of sliders in jumpDistance.
• Uses slider velocity as a scale to determine how hard a slider is to read.
  • This nerfs some sliders, but mostly buffs long and fast sliders.
• Apply hidden bonus to sliders.

Change examples:

• -GN +FL on Kikoku Doukoku Jigokuraku [Notch Hell]: 406pp -> 546pp
• hurrikate +HDFL on Der Wald (Kors K Remix) [Maze]: 131pp -> 259pp
• WaterKiller +HDHRDTFL on Kona-Chan: Farucon Pan! (sm2164009) [Insane]: 198pp -> 231pp

This change also fixes a silly bug where spinners' StrainTime was not being accounted for correctly.

Example:

• molneya +HDHRNCFL Boukoku Kakusei Catharsis (TV Size): 621pp -> 542pp

[molneya]

Closing since there hasn't really been any movement in the pp scene for a while, and I've got a new branch with more changes that I'm working on instead.