Refactor the IProjectFilter interface for threading usage.

Instead of implementing the IProjectFilter interface, a plugin
now implements a IProjectFilterProvider interface. It comes with
a simple factory function to create a threadsafe IProjectFilter.
This is supposed to be used with value-semantics. All initialization
should be done in the ctor of its implementation, such that later
calls to isValid do not introduce any race conditions.

When a user changed any filters, the Provider emits a filterChanged
signal. Project managers and other users have to listen to it and
update their IProjectFilter instances then, i.e. replace their
existing instance with a new one as returned by createFilter.

This should now allow this interface to be used in the CMakeManager.
Furthermore it should be a future-proof design if we ever get around
to implement a proper API for threaded project import.

Note: If you think this design is complicated, then yes - the old
one was much simpler. This one has a huge advantage though of not
requiring any locks and thus leading to much better scaling when
we import from a background thread.