optional ports in QT-GUI blocks

[mbr0wn]

Currently, qt gui blocks have an optional stream port and message ports hidden at the same time.
This works, but kind of looks like a bug to some users.

Right now, the tag is static, so I propose to enable Cheetah eval in there, so the stream ports can be mandatory if message ports are hidden.

(also, must be one or up - '0' gets converted to one")