Мат в 2 хода. Шахматные задачи

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* Invert board option is replaced by on-board gesture
* Added "Auto next" option - automatically switch to the next exercise
* Added UCI engine support
* Added ability to navigate through lesson using swipe gestures on notation
* Added chess symbols convention
* Added "Copy PGN" option
* Added "Clear statistics" option
* Improved current theme and subtheme storing
* Improved test mode
* Various fixes and improvements