A ridiculously large commit as a checkpoint in refactoring the time_t + an int

A ridiculously large commit as a checkpoint in refactoring the time_t + an int
for microseconds into a QDateTime to give us sensible interface to time
beyond 1970 and 2032 and, more topically, more unified handling of sub-second
time now that 10Hz GPSes aren't uncommon.

This gets almost all of the accesses to wpt->creation_time into accessors
(with overloads for int/time_t and QDateTime) and eliminates many of
the problematic direct accesses to wpt->microseconds.