Add command key to "Pause" plotting but leave memory allocated

[XCHSystems]

During the copying from my Optane SSD to spinning disk, as the spinning disk starts to fill up the copy process starts to slow down, which results in more plots waiting to be copied via rsync, this can lead to a full Optane SSD and plot creation failures, if I stop Bladebit, then all buffers get allocated to the copy process, which means when I re-launch bladebit, I have to sync and clear all buffers.

If there was a Pause command key in Bladebit that paused the plotting in the current status, but left the memory allocated, it would allow the copy process to catch up and clear the fast SSD