Add Outputbox clearing options (#2)

[Json-To-String]

I tried having a go at #2 . This adds

1. a context menu option to clear the whole textbox
2. a return/enter shortcut to add a newline
3. a ctrl+L shortcut to jump to the end

Although reading the issue again

Maybe also it should be that if you hit control-L, the window scrolls down past the last line so that the next printed line will be at the top.

Was this more intended to force a scroll and keeping the previous text to scroll back to later rather than clearing down to the last line? In which case 3. isn't quite right since it'll discard the entirety of the output and displays just the last line at the top, although I believe the built-in QT functions might be easier to leverage for the former.