Task-number: QTCREATORBUG-807

Main reason is that find-usages and refactoring broke completely.
Second reason is that concurrent modification was done on the
children-list while deleting/destructing.

This reverts commit b0b95f88.

Patch by: Bill King

Encapsulate the details.

Unmark the visited nodes when searching bindings in class or namespace scopes.

Reviewed-by: Trust Me

Details:
This patch makes use of system icons as a base
and draws overlay on top. This should blend a bit better
into the native icon theme.

Many singletons have their own instance() method, in which case there is
no need to use ICore::instance() to get access to them.