Change playback speed exponentially

[mklaber]

This PR closes #729

This PR addresses an issue identified in #81 and alluded to in #254: speeding up playback via the keyboard at an exponential rate. Unlike the original PR, #729, this PR does this by changing the existing playback rate functionality to use an exponential rate (∛2). Additionally, it adds a SHIFT modifier key to the existing J and L commands which increases the rate-of-change even more. When in use (by default) a series of L presses will go from stopped → playing at 100%, 126%, 158%, 200%, etc., and up to 1600%. J will go from stopped → playing at 100%, 79%, 62%, 50%, etc., and down to 10%. When the SHIFT key is pressed, it uses a factor of 2 (100%, 200%, 400%, ...).

• Adds a SHIFT modifier to L and J
• If speeding up or slowing down (whether using the modifier or not) brings the user from slo-mo to sped-up (or vice-versa), it makes sure to stop at 1x/100% along the way. (This allows the user to use a combination of multiplied speeds without getting stuck unable to return to real-time.)
• Adds documentation for the new commands to the HelpSheet

At some point in the future, if the playback rates selected prove to be unpopular, a setting can be re-introduced (as was done in #729) that allows the user to specify their own rate.

[mifi]

Sorry for the delay, I just needed some time to properly go through this. Really awesome work, thanks! I'm merging this now, hopefully nobody will complain that the default speed-up is too fast now.