Add semantic integration copy actions

• Selecting Copy (or pressing ctrl-shift-c) when there is no selection selects current output if current mode is output, current input if current mode is input and input is non-empty, and last output if current input is empty.
• Ctrl + mouse triple click selects the pointed input/output/prompt.
• There is also a fix that was missed in previous patch for keeping track of current semantic mode through scrolls.

I believe this answers the last remaining point from !691 (comment 480025).