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

[rerpha]

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

[rerpha]

consider if not hugely urgent making this a training ticket (maybe even good first issue) - should be reasonably straightforward.

[rerpha]

maybe write prefix in italics when local box checked