use std::chrono::microseconds for time in input events

There's no reason to use milliseconds or even both milliseconds and microseconds (as done in some places), the millisecond time function of libinput is only there for backwards compatibility.

Commit 1: remove properties from SeatInterface. They don't seem to be used or needed for anything

Commit 2: Port time of input events to std::chrono::microseconds

Commit 3: Fix timestamps in FakeInputDevice