Add the Timezone Offset to Extra Info.

Add an 'Extra Info' field to show the timezone offset in seconds for
dive computers that supply this information.
Essentially a 'cut out the middleman' for
subsurface/libdc#100.

Signed-off-by: Michael Keller <github@ike.ch>

Author: mikeller

core/libdivecomputer.cpp

@@ -650,6 +650,11 @@ static dc_status_t libdc_header_parser(dc_parser_t *parser, device_data_t *devda
 		tm.tm_min = dt.minute;
 		tm.tm_sec = dt.second;
 		dive->when = dive->dcs[0].when = utc_mktime(&tm);
+
+		if (dt.timezone) {
+			std::string timezone_offset = format_string_std("%+d", dt.timezone);
+			add_extra_data(&dive->dcs[0], "Time offset from UTC [s]", timezone_offset);
+		}
 	}
 
 	// Parse the divetime.