GUI: add highlighted text when "local" box is ticked when adding block

As a user I would like the `local` box, which prefixes the instrument prefix (ie IN:LARMOR:) to a pv address when adding a block to give more feedback as to what it's doing.

This could be in the way of adding highlighted text showing the FULL pv address, that updates when the `local` tickbox is changed. ie when local is ticked the pv address of a block could be:

when local is unticked the pv address box removes the prefix and the pv address textbox then contains:

### Acceptance criteria
- [ ] When a user selects `local` when adding a block, the prefix is appended in the textbox for "PV address" and some indication is given that the local box is influencing this (be it highlighting, coloured text etc.)
- [ ] When a user unticks the "local" box, if there is a prefix which matches the instrument prefix, this is removed from the "PV address" textbox
- [ ] Squish tests are added for both cases