Integration of the new splittable tab widget of pyqode.core

Integrates the new splittable tab widget of pyqode.core into the IDE. 

Here is how it looks: http://youtu.be/0oK--lOdYiI

To test it on linux, just run the following commands:

```
pip3 install git+https://github.com/pyQode/pyqode.core.git@develop --upgrade
pip3 install git+https://github.com/OpenCobolIDE/OpenCobolIDE.git@feature/splittable_tab_widget --upgrade
```

You can report any bugs you encountered while testing directly here.