Implements managers and refactor namespaces

I recently made quite a lots of changes to the public API. The goal was to make the public API more pythonic and consistent.

What changed ?
---------------------

- frontend package removal: all the modules and subpackages have been move to pyqode.core (parent package)
- move pyqode.qt into pyqode.core
- add managers: a manager literally manages one specific aspect of CodeEdit, e.g. there are managers for modes and panels, for managing the backend, for loading and saving files.  Instead of having a set of free functions in the frontend pacakges, all those functions has been moved to classes and have been tightly integrated into CodeEdit.

I am submitting this as a PR so that people who are already using the 2.0-beta code know that they will have to update their code. I am sorry for that but those changes had to be done before the final public release. I hope those changes will make the API easier to use!

Here is how the basic.py example now look like:

``` python
import sys
from pyqode.core import api
from pyqode.core import modes
from pyqode.core import panels
from pyqode.core.qt import QtWidgets

def main():
    app = QtWidgets.QApplication(sys.argv)

    # create editor and window
    window = QtWidgets.QMainWindow()
    editor = api.CodeEdit()
    window.setCentralWidget(editor)

    # start the backend as soon as possible
    editor.backend.start('server.py')

    # append some modes and panels
    editor.modes.append(modes.CodeCompletionMode())
    editor.modes.append(modes.PygmentsSyntaxHighlighter(editor.document()))
    editor.modes.append(modes.CaretLineHighlighterMode())
    editor.panels.append(panels.SearchAndReplacePanel(),
                      api.Panel.Position.BOTTOM)

    print(editor.modes)   # modes and panels are now iterables!
    print(editor.panels)

    # open a file
    editor.file.open(__file__)

    # run
    window.show()
    app.exec_()

if __name__ == "__main__":
    main()
```

*ps: those changes have already been ported on pyqode.python. I will release a last beta once this has been merged.*