Integration of the new splittable tab widget of pyqode.core

(Requires pyqode.core@develop)
Fix segfault when closing the application after a tab has been split

Also improve new split size (do not reuse previous tabs)

THIS REQUIRE TO UPDATE pyqode.core TO THE LATEST commit on the master branch
Fix a circular reference that cause the app destructor from not being called

This needs to be tested on Windows
Fix tests, pyqode does not use a develop branch anymore

(and OpenCobolIDE will also drop it soon, after the next release)
Merge pull request #75 from OpenCobolIDE/feature/splittable_tab_widget

Add a splittable tab widget to the editor page