control flow plugin reloaded

[David Nolden]

Before the plugin is moved into kdevplatform, it has to satisfy a few criteria:
1. It should not do anything when its view is not visible
2. All of its operations must be interruptible
3. It shouldn't crash (don't use Declaration* over duchain locking
cycles, check for validity of DeclarationPointer values, or even
better use DeclarationId, etc.)

I don't know how far it complies to these criteria, but especially 1.
and 2. are required to make it acceptable (point 3 would be easier to
fix). My feeling of the time of development was that it was mainly
tested on "toy" projects, but on larger projects these points are very
critical.

Greetings, David