view: fix wrong currentStone position when starting dragging
currentStone: don't move stone back when started from below

when long pressing on a stone and the finger moves sideways,
don't take the stone back onto the wheel even though the end
point is below the field
refactoring: make ViewElement an interface, make ViewElements executable
wheel: damp spinning of wheel when swiping vertically. wheel snaps back when start dragging
wheel: always show current and highlighted stone when dragging
hints: fix position bug introduced with recent commit about currentStone position
wheel: don't animate wheel when animations not enabled
view: reduce FPS to 5 when animations are disabled
little refactoring, move RetainedConfig to separate class