The follow-up to e2457418da, which
replaces most of the includes in the core header with forward declarations.
This makes it so that if any of the headers the core header was
previously including change, then no one will need to rebuild the bulk
of the core, due to core.h being quite a prevalent inclusion.
This should make turnaround for changes much faster for developers.
25us is far too small, and would result in std::this_thread::sleep_for
being called with this as a maximum value. This means that a guest
application that produces frames instantly would only be limited to
40 kHz.
25ms is a more appropriate value, as it allows for a 60 Hz refresh
rate while providing enough slack in the negative region.
Now based on std::chrono, and also works in terms of emulated time
instead of frames, so we can in the future frame-limit even when the
display is disabled, etc.
The frame limiter can also be enabled along with v-sync now, which
should be useful for those with displays running at more than 60 Hz.
