Added `SDL2TicksProvider` header file with... `SDL2TicksProvider` class extending `TicksProvider`
Added definition of `SDL2TicksProvider::ticks`
Included `SDL2TicksProvider` header
Mistake: include `SDL2TicksProvider`, not `SDL2Window`;
Mistake: no type of `SDL2TicksProvider::ticks`; Now is `time_t`
Provided second argument for `Game` base class -> `new SDL2TicksProvider`