Provide explicit constructors/destructors set

* Explicit destructor is needed to remove list of GameObjects, header
  for which is only included in game.cc file
* Copy ctors are explicitely disabled
* Move ctors are explicitely defined as noexcept, as not all compilers
  can deduce that automatically (see comment in game.cc)
Rename viewer -> gfxviewer
Add map viewer utility
Update README
Fix Game::GetHeight()
Fix viewport and scrolling in map viewer