[3.0] add vlc_tick_t type and defines to the 3.0 branch

It's becoming trickier and trickier to backport patches from 4.0 without hitting changes that only affect mtime_t vs vlc_tick_t differences.

It's better for the future maintainability of 3.0 to allow using vlc_tick_t and still keep mtime_t for external code.

This MR backports the 3 main patches than switch the code usage from vlc_tick_t to mtime_t. It doesn't change the mdate() calls nor includes the fancy conversion macros/inline functions found in 4.0 (this may come later).

!2443 (merged) is an example of MR that benefits from this change, almost not patch has been edited when cherry picking.