- 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