(This commit also modifies UI)
Adds a universal function for determining whether video output is
currently active, rather than having to use video_output_active() on the
value returned by obs_get_video().
When building with VS2017 the compiler shows some warnings which is
undesirable.
The compiler rightly complains about declarations which hide older ones
which are trivially fixed.
Finally only POD types should be passed to variadic functions which is
why the String should be converted to c_str().
Relying on the current undefined behavior is a bad idea as it can change
with compilers and compiler versions.
Closesjp9000/obs-studio#902