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.