Fix FrameTime internals

[mgoral]

Replace full_seconds (float) with milliseconds (int).

Initial implementation: https://gist.github.com/mgoral/091c3899d1654a79be517ec0c2dbc53e

[mgoral]

Currently there's a bug, which causes calculating 1000 milliseconds in some circumstances. The following test fails for example:

@pytest.mark.parametrize('secs', [(123.9989999999, 0.001),
                                  (-123.9989999999, -0.001)])
def test_ms_rounding(secs):
    lhs = FrameTime(25, seconds=secs[0])
    rhs = FrameTime(25, seconds=secs[1])
    ft = lhs + rhs
    assert ft.time['miliseconds'] == 0

Using milliseconds as integer should fix the problem, because no rounding will be necessary.