Add a SplittableTabWidget to pyqode.core.widgets

Implements #148

The new tab widget has been integrated into the pyqode.core and pyqode.python application examples (notepad and pynotepad).

Testing it is easy:
``` bash
git clone https://github.com/pyQode/pyqode.core
cd pyqode.core
git checkout feature/splittable_tab_widget
pip install -e .
cd examples/notepad
python notepad.py
```

Before I merge this I will first have to integrate it into [OpenCobolIDE](https://github.com/OpenCobolIDE/OpenCobolIDE) (to make sure the API is as feature complete as the TabWidget API).