OS time functions not implemented

[Tarudahat]

Godot version:
3.2.1-windows

Issue description:
OS functions like "OS.get_system_time_secs()"& "OS.get_system_time_secs()"
don't seem to be implemented. There might be more but I havn't tested it.

Temporary fix:
Cap fps and count the frames to get a sec.

[Stary2001]

Whoops - seems I missed this when sorting out the release.
That makes sense, as it seems get_date/get_time/get_time_zone_info are still stubbed out.

OS::Date OS_Switch::get_date(bool local) const {
	return OS::Date();
}

OS::Time OS_Switch::get_time(bool local) const {
	return OS::Time();
}

OS::TimeZoneInfo OS_Switch::get_time_zone_info() const {
	return OS::TimeZoneInfo();
}