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