Factor out parse_datetime()

Create a new function that takes a time stamp and updates the date or
time components of that time stamp according to a given date/time
string. Use that function for updating the start time of an item.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/calcurse.h

@@ -1136,6 +1136,7 @@ int check_time(unsigned, unsigned);
 int parse_time(const char *, unsigned *, unsigned *);
 int parse_duration(const char *, unsigned *);
 int parse_date_duration(const char *, unsigned *);
+int parse_datetime(const char *, long *);
 void file_close(FILE *, const char *);
 void psleep(unsigned);
 int fork_exec(int *, int *, const char *, const char *const *);