Revert "Enforce setting focus to the document tree view when opening a document"

This patch breaks a common workflow, where switching between files,
then pressing "ctrl + c" to save the selection of a file, actually
copies the file not the selection in the editor.

If you want to select something in the editor in the single-mouse-
click environment, then use CTRL + click.

CCMAIL: niko.sams@gmail.com
CCMAIL: kdevelop-devel@kde.org

This reverts commit 782ce8b5.