This commit makes `get_current_time_msec` correctly return milliseconds
as opposed to microseconds. It also considers the value of `tv_sec`, so
we don't lose occasionally go back in time by one second. Finally, the
function is moved into `util/time.cc` so that it can be reused elsewhere
without having to consider these pitfalls.
Previously, each time a wl_seat.capabilities event was received the
Wayland backend created new input devices. It now only does so the first
time.
Input devices are now destroyed when the cap is removed.
Closes: https://github.com/swaywm/sway/issues/5055
