enhancement: selection should copy to PRIMARY selection buffer

[eMBee]

based on the terminology used here: https://unix.stackexchange.com/a/84251/137741
there are two relevant selection buffers: PRIMARY and CLIPBOARD

PTerm is very convenient in that it can paste by right click from both. however, taking a selection from CLIPBOARD always overrides a selection from PRIMARY.

on X window i primarily use PRIMARY because it works with mouse only, not needing a keyboard.
copying from X to PTerm works fine. however, as soon as i select something in PTerm, this is copied into CLIPBOARD, and i can no longer use simple selection in X to copy from X to PTerm.

instead i have to either use Ctrl-c on X, or a special tool that allows me to clear the CLIPBOARD buffer.

also, middle-click is never enough to paste something from PTerm into X. i always need to use ctrl-v

i see two ways out of this: in PTerm always only use PRIMARY just like in X, unless shift-ctrl-c and shift-ctrl-v are used. this would be consistent with every other terminal which all operate like this.

a selection copies to PRIMARY, and only shift-ctrl-c copies to CLIPBOARD
right-click always only pastes from PRIMARY, and only shift-ctrl-v pastes from CLIPBOARD

alternatively, let a selection copy to both so that it works either way in X, track which content was last in each buffer and then let right-click paste from the buffer that has changed. this should allow right-click to always pick up the latest selection made outside pharo, whether it is done through mouse selection or ctrl-c.