Rewrite for 4.0

For the next version, I decided to rewrite the application nearly from scratch. There are many reasons why this is needed:

   - the previous code is a mess. It is poorly organised and does not follow any convention
   - I have to write more and more pyqode related features to improve the cobol editor widget: code folding, native syntax highlighter,... I want to move all that code to pyqode, by creating pyqode.cobol.
   - issue #47 implies a review of the main window UI and I want to use this opportunity to do it well.

To finish this pull request, I have to:

  - [ ] move the cobol editor widget to pyqode.cobol
  - [ ] rewrite the application code:
       - [ file management
       - [ editors and view management
       - [ compiler management
       - [ settings management
       - [ perspective switching (#47)
  - [ ] polish the user interface (things like tooltips,...)
  - [ ] improve test suite coverage
  - [ ] improve naming convention by closely following PEP8 guidelines