Add support for UTC times in iCal imports

According to the iCal standard (4.3.12 Time):

    UTC time, or absolute time, is identified by a LATIN CAPITAL LETTER
    Z suffix character (US-ASCII decimal 90), the UTC designator,
    appended to the time value.

Parse such time values properly when importing iCal files.

Fixes GitHub issue #3.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -1054,6 +1054,7 @@ long get_item_time(long);
 int get_item_hour(long);
 int get_item_min(long);
 long date2sec(struct date, unsigned, unsigned);
+long utcdate2sec(struct date, unsigned, unsigned);
 char *date_sec2date_str(long, const char *);
 void date_sec2date_fmt(long, const char *, char *);
 long date_sec_change(long, int, int);