VCS: Get current file from the editor

VCS: Get current file from the editor

Get the current file from the editor, not the filemanager. The later
does not get updated for temporary files like those created by git
blame, leaving the file-based actions in the VCSes enabled when viewing
those temporary files, triggering actions on the last real file that
was visible.

Task-number: QTCREATORBUG-6250
Change-Id: I5c2d3c62805f69bc8e103f0776cd6796332f606c
Reviewed-on: http://codereview.qt-project.org/6175
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>