Makes the Code Editor Widget tabs closable.

Summary:
Making the code editor widget tabs closable by creating a
method that receives a signal from the tabCloseRequest, closing the
specified tab and removing it's code document properly.

Reviewers: tcanabrava

Reviewed By: tcanabrava

Subscribers: cordlandwehr, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D15990