Append the string to the buffer.
Only use them on normally allocated memory, or on buffers created from icalmemory_new_buffer, never with buffers created by icalmemory_tmp_buffer. If icalmemory_append_string has to resize a buffer on the ring, the ring will loose track of it an you will have memory problems.
The buffer to be appended. It should not be the memory in ical.
The position at which the new string to be appended.
The string to be allocated
The size of the buffer before appended