[GSoC 2020] Particaly realization of T9475 with some minor changes:
- Add new GraphicPackage class for working with external graphic packages in different backends - Add graphical package support in Python backend. On this moment supports matplotlib, seaborn, pylab, plot.ly, ggplot. - Add new signal "expressionFinished", which Cantor::Expression will emit, when goes to Done, Error or Interrupt states - Some minor code style cleanup with spaces and tabs - Minor documentation adding