[brd] Add zone net selection indicator and fill gui interface

[abetusk]

Right now selecting zone net selection is done by hitting the 'z' key multiple times and when placed, selecting and hitting 'z' again to fill. This at the least should tell you which net (and layer) the zone boundary is for somewhere. There should also be some selectable gui widget to indicate which layer to pick.

• Display net name and layer associated with the selected zone (maybe lower left in the text marquee?)
• Expand 'clickable' bounds so the zone is easier to select
• Double click to put it into edit mode?
• Give a floating dropdown that you can select which net to associate the fill with. Highlight as you hover/scroll through the nets.

priority:low
estimate:5

[abetusk]

priority: high
estimate: 5