Buffering in GUI

[Chao1009]

The previous method to read a DST file and display in the GUI is not valid for a very large file, since the GUI will read all the events into memory.

With the recent updates for the DST file, it now has an event map, which enables the random access to any event in the file.

So it will be straightforward to have a buffer for only reading a small amount of events from a DST file, and updating itself when the visiting event is out of the buffer.