Computation of Exclude/Include patterns in a non-persitent way by an extension

Currently I can define exclude/include items from classpath by specifying a pattern:


this is then stored in the `.classpath` like this:

``

What I like to archive is, that this value is not persistently stored in the classpath, but computed by an extension point, such an extension point should get the `IClasspath` element (and if possible the `I(Java)Project`) and would return then include/exclude patterns that should apply to this classpath element.

@stephan-herrmann what do you think? This would be a more generic replacement of the enable/disable, and actually we already use this in m2e, but only compute it once and store it in the `.classpath`, this would also help with:
- https://github.com/eclipse-tycho/tycho/issues/632

PDE can simply then implement this extension and filter out all classes that do not match the current target environment.

One problem I see is that when to actually this must be applied, e.g. when resolving classpath? but this is also shown in the UI as "disabled", so the UI must probably also call the extension point.