Enhancement : Expand outline nodes with a double-click

The content outline nodes of the master-details editor page should
be able to expand and collapse in response to a double-click.

Bugzilla: https://bugs.eclipse.org/bugs/show_bug.cgi?id=455493
Signed-off-by: Dmitry Karpenko <dmitry.karpenko@onpositive.com>
Releng : EGit 3.6