DSP external filter crash

[ustk]

So I might not use it the way it should, but:

• When trying to connect a DSP through the ProcessorId of a floating tile FilterDisplay, applying the change makes an instant crash

• Add new external filter in a DSP filter doesn't propose more slots#, so only one is made (but again I might not use it the right way)

@Christoph-Hart Should I push the issue?

[Christoph Hart]

@ustk Yes please.