Add pause button to GUI log

GUI log widget jumps to end when stuff is appended to it, making it impossible to scroll while active. Maybe add a pause ~button~ _checkbox_ to the window, so as to buffer the new lines while paused and flush them at once when unpaused?

Locate the GUI log widget and trace how appended lines currently force the view to the end. Add a pause checkbox that buffers new lines while paused and flushes them when unpaused; done means the log can be scrolled during activity and buffered output appears after resuming.

Written by the indexing model from the issue text.