Various zone editor fixes

[meronz]

• Improve editor styling with round ZoneAnchors and rounded corners for zones.
• Fix "Cancel editing" not forgetting changes.
• Resize label now shows the inner dimensions (so with margins taken into account).
• Fix editing on multiple screens (#40)
• Introduce minimum width/height for zones
• Fix bad widget size warning.

There are a few bugs remaining, but this branch compiles and it's mostly working for anyone to try. To be fixed:

• [x] ZoneAnchor button is shown "below" the zone widgets.
• [x] ZoneAnchors do not immediately show when going in edit mode.

[rsaffi]

As mentioned here, the fixes from both #54 as well as #40 worked for me!