Draft: qt: make `VLCTick` constructor explicit, add explicit floating point and signed integer constructors

See individual commits.

This should make it clearer and easier to handle 0 (VLC_TICK_INVALID) special case. We can directly use unsigned integer constructor and treat 0 normally (vlc_tick_t is a int64_t alias). At the same time, having an explicit floating point number constructor makes it possible to take care of NaN.

I think this should satisfy what we have discussed @chub.