Target FPS has several types and produces warnings

[Maykeye]

Describe the bug
When compiling with a compiler which frowns between float<->uint32_t implicit conversions, code produces warnings because target fps is float in some places, integer in other:

• flecs.hpp:world::ecs_set_target_fps expects std::uint32_t fps target_fps
• flecs.hpp:world::get_target_fps returns std::uint32_t
• flecs.h:ecs_set_target_fps expects float fps
• flecs.h:ecs_get_target_fps returns uint32_t

To Reproduce

1. Compile in VS2017 c++ file with `#include <flecs.h>
2. get >e:\work\vs2017.libs\flecs\include\flecs\flecs.hpp(361): warning C4244: 'argument': conversion from 'uint32_t' to 'float', possible loss of data

Expected behavior
Same types everywhere with no warnings

Additional context
Related to #70 (compiler warnings with Visual Studio 2017), but if #70 it's one time problem - you compile the library and forget about it as you use compiled lib, here warnings produced every time you include the header in actual projects.

[SanderMertens]

Thanks for reporting! I'll change the type to float everywhere.