mac: only update touch bar items when necessary

the slider on the touch bar was always updated when any of the related
properties changed their value. this is partially dependent on the
refresh rate of the video, in the case of time-pos. too many updates to
touch bar impact the render performance.

to prevent this we only update the slider when necessary, when the touch
bar or the touch bar item is visible. the touch bar items only need a
granularity of seconds without any decimals, but the time-pos property
provides a granularity with decimals. we floor those values and only
update the touch bar items when we have at least a 1 second difference.
we also check for the visibility of the touch bar and its items.

Fixes #8477

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