Buffer count management - GUI support

With CH-161 the async io was introduced. With that change, the buffer management also changed. Previously the buffer sizes specified in the buffer dialog were used to allocate a single buffer for data which was used for reading and then writing data.
With the new mechanism, there are N buffers allocated of the size specified in the Buffer Size dialog. N is an ini option at this point.
What is missing is the possibility to specify the count of buffers in the Buffer Size dialog.