Extend the editor sidebar background to the folding markers

Extend the editor sidebar background to the folding markers

Previously, there was a gab between the text and the sidebar which
contained the folding markers. This is disturbing for some users.

Change-Id: Ib3c90d9cb01de533d16049bc1a0f5f10f1f7847a
Task-number: QTCREATORBUG-2915
Reviewed-on: http://codereview.qt.nokia.com/355
Reviewed-by: Alessandro Portale <alessandro.portale@nokia.com>