Add support for game variations

[herzbube]

With the intregration of SgfcKit the app has now access to game variations that are stored inside an SGF file. The app should be able to display game variations, but also to create new game variations and modify/delete existing game variations.

[herzbube]

Search the code for "variation support" for markers where it is already known that the code needs to be changes to add variation support.

[herzbube]

Merge commit e9eea6a7612c52f53642148182f5e0905956ad9d contains a first set of important changes:

• Changes to the data model, the most important of which are
  • Store setup information in GoNodeSetup
  • Move property zobristHash from GoMove to GoNode
  • Remove GoMove reference to the next move (not possible in a tree)
• Loading of the full tree of SGF nodes in LoadGameCommand
• Cleanup: GoBoardPosition no longer uses KVO to react to changes in GoNodeModel
• Added unit tests for GoNode which were missing until now

[herzbube]

Merge commit 035304eb661071251d9f457036c0e1a0cba1fa20 contains the next batch of major changes:

• First working implementation of the scroll view that displays the node tree. Major parts/features:
  • Calculate the view model data from the Go node model data.
  • Draw node symbols, lines between node symbols, the currently selected game variation, and the selected node.
  • Zoom in/out of the node tree view.
  • User can tap in the node tree view to select a node => this changes the current game variation in GoNodeModel!
  • When the current board position changes, the node tree view focuses on the newly selected node .
  • A set of 4 new user preferences that govern how the node tree is drawn.
• The app now remembers which was shown last: The list of board positions or the node tree view.
• Several bugfixes in LoadGameCommand.
• Several game variation handling fixes (GoNodeModel discard methods, ChangeAndDiscardCommand, SyncGTPEngineCommand).
• Other improvements too numerous and detailed to list.

[herzbube]

Further important commits:

• 374e29ea4891bd2db039dca6043b7f6fac71616e: new move creates game variation instead of discarding future board positions
• 5fd272dd2baecbd42ebb5574045a3227f30b6b18: new user preferences "new move insert policy" and "new move insert position"
• c52a623842cc2603157f0ce245be19961b0d6b42: new user preference "focus mode"
• e8ab1e6f81c4b95527268cc966028695d98ce541 - 2f0acb963b8eeeb99d290990fa3b53be7bff50d5: display node numbers

At this point rendering the node tree seems quite complete. The development focus now shifts back again to integrating the view into the device-specific UI layouts.

• 4086fc2b686c4057b98b932e321ded7e6a5992cf: drag handle drawing is now more sophisticated
• abc252550ce4a22a0d314dc50d479dd011e0b4e5: size distribution of board view / node tree view is now a user preference

[herzbube]

In the year since the last comment, the work on this feature has been finalized, bugs were eliminated and unit tests were written. Overall it's too much effort to link all the relevant commits to this feature.