Refactor the Solver class

Summary:
This is a set of 3 changes:

1. Simplify the structure of the solver class:
   • make firstMoves and winMoves private members and add getter for them
   • replace mutex protected bool with atomic bool
   • do not directly expose m_shouldEnd, but provide a stopExecution function
   • use in-class initialization of member variables
   • remove debug variable, and the DEBUG_HINTS define which controlled it, use #ifndef NDEBUG to print the debug messages for now
   • Solver: use a unique_ptr for the MemoryManager

2. Introduce a smaller SolverInterface, hiding the complexity of Solver

This change:

• extract all the publically used Solver functionality into SolverInterface, which consists only of virtual functions
• move solverinterface into its own header, to minimize includes
• converts users which used Solver directly to SolverInterface

3. Convert Solver into a template, templated over the number of piles
   • The number of piles is known in each Solver subclass at compile time, thus a subclass with n piles does now inherit from Solver<n>.
   • m_number_piles and its setter become redundant with this change, and are removed
   • This enable the conversion of member variables which were dynamically allocated arrays into std::arrays, increasing type safety and causing an abort/exception on out-of-boundary access. Furthermore, the compiler can now better reason about some of the loops (though this doesn't really matter performance wise)

Test Plan: Everything still compiles, and the Solver can still solve each game

Reviewers: KDE Games, aacid

Reviewed By: aacid

Subscribers: aacid, KDE Games

Differential Revision: https://phabricator.kde.org/D11815