Add scrubber/slider control to quickly move between board positions.

[herzbube]

Tasks:

• Make a quick & dirty prototype to see what this means performance-wise.
  • Changing many board positions in a short time is a potential CPU hog because each step can cause a lot of redrawing, and the dynamic GoBoardRegion calculations are also terribly inefficient.
  • Measure on a real device.
  • Measure whether there is a difference when some settings are turned on, notably "Mark last move" and "Display move numbers".
  • Concerns may turn out to be unfounded because modern phones are so much more powerful than when development of the project started for the iPhone 3GS 10 years ago. Nevertheless, it's better to know than to hope.
• UI design
  • Find out how to integrate this control into the three UI types.
  • See if a non-basic slider design is feasible without too much effort. Something like this slider (taken from the app "Pandanet Go") would be nice.
• Implementation
  • Take hints from the way how the "rewind to start" and "forward to end" buttons are implemented.
  • The slider must also update itself if the board position is changed with other means.

[herzbube]

Issue #113 has hints about optimization potential.