partial cleanup of include files in gui sources * symbol-information.cc, octave-adapter/octave-event.cc: New files. * documentation-dockwidget.h, history-dockwidget.cc, m-editor/file-editor.h, octave-event.cc, octave-event.h, octave-link.cc, octave-link.h, octave-main-thread.cc, octave-gui.cc, symbol-information.cc, symbol-information.h, workspace-model.cc: Avoid including unnecssary header files.

Copyright notices are intentionally omitted from the files in the
examples directory.  We don't want copyright notices cluttering the
manual.