Document tree view close on middle button

Summary:
KDevDocumentView::mousePressEvent handles middle button over
document to close them quickly.

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: anthonyfieroni, kossebau, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D22160