Add a SplittableTabWidget to pyqode.core.widgets

The SplittableTabWidget is a tab widget whose tabs can be split infinitely. Any widget may be added to the tab widget as long as they implement a split method. There is an extension maded specifically for holding code editors. This extension provides convenience methods for file management.

There is support for drag & drop: you can drag tabs inside a tab widget or between the different splits. This let you reorganize your workspace the way you want.

Support has been added to the various CodeEdit (including pyqode.python and the newly create pyqode.json). All settings are automatically transferred from the original editor to the clones and the clone chained is automatically adapted if the user close the original editor (the first clone is then considered as the original).
Fix a few bugs and improve API for integration into OpenCobolIDE
Merge pull request #150 from pyQode/feature/splittable_tab_widget

Add a SplittableTabWidget to pyqode.core.widgets