Introduce prof_date_time_format_iso8601().

Instead of repeating the same pattern over and over, introduce a helper
function that either outputs a formatted timestamp if one is given as
argument or returns the current time if no argument is given.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

src/common.h

@@ -187,6 +187,8 @@ gchar* get_expanded_path(const char* path);
 
 char* basename_from_url(const char* url);
 
+gchar* prof_date_time_format_iso8601(GDateTime* dt);
+
 gchar* prof_get_version(void);
 void prof_add_shutdown_routine(void (*routine)(void));