Writing this specification unveiled a limitation of our internal
time base implementation, which is a 64bit microsecond grid.
As it turns out, any grid based time representation will always
be not precise enough to handle some relevant time specifications,
which are defined by a divisor. Most notably this affects the precise
display of frame duration in the GUI, and even more relevant,
the sample accurate editing of sound in the timeline.
Thus I decided to perform the internal computation in ZoomWindow
as rational numbers, based on boost::rational
Note: implementation stubbed only, test fails
