Dropdown for topic should update

[mitra42]

The dropdown for a topic (on a control object) is defined when the discovery message comes in.

• However, they arrive in random order - and the dropdowns should be dynamically populate whenever another topic of the appropriate type is discovered.
• The dropdowns also include irrelevant topics
  • - in particular should not for example populate a drop-down with another control (rw=r)
  • should be dependent on rw which may need extension to options
  • should not wire to self e.g. output of control hysterisis can be wired to its own output
• Dropdowns shouldnt start off "unused" if its actually wired to something already (poss just listening to that topic will work)