Before, we jumped to a symbol as soon as the selection changed, which
was problematic:
 * It was inconsistent to other outlines (e.g. class view, project
   explorer)
 * Using the Up/Down keys in the sidebar immediately jumped to the
   symbol in the editor, thus polluting the editor history.

Now we jump to a symbol if the corresponding item was explicitly
activated (QAbstractItemView::activated() signal).

There was also another strange issue: If "Synchronize with Editor" was
de-activated, nothing happened upon activation/selection of an item. Now
"Synchronize with Editor" means to update the selection in the side bar
if the cursor position changes in the editor (one direction only).

Task-number: QTCREATORBUG-12412
Change-Id: I8d9191d5fa8e229723194dcf30081e144debecbb
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>