General Cleanup

I tried to get back to refactoring mentioned in an old merge request !80 (closed). I've borrowed a lot of code from there, still this might not be all, it's a start :). If you think that it would be better split into smaller merge requests, let me know.

Let me reiterate what was said there:

• Made all narrowing conversions explicit with static_cast or changing the types
• Removed a whole bunch of unnecessary memory allocations
  • constexpr char[] instead of global std::strings
  • std::move and `emplace_back
  • range-for
• General modernization
  • NULL -> nullptr
  • std::list -> std::vector (closes #37 (closed))
  • Don't use delete, use RAII
  • STL algorithms
  • References instead of pointers
  • Use more standard library types ()

Tested it inside amd64 qemu both as a package after logging in and using pmbootstrab install --add osk-sdl --fde and it seems to work fine, unfortunately I don't have any device to test it on (yet!). The only issue that I've run into is that while running with --fde on-screen keyboard can't be used. Although I've heard that it's a known problem.