Make layer / sub-layer labels conditionally editable by the user

[prushforth]

For discussion

It would be nice to allow the user to edit the label value (unless there's an in-scope <map-title>) and have it hold for the lifetime of the user session. Probably that means that a <map-title> element inside the <map-extent> is necessary, so that the map author of the mapml document could make the sub-layer label non-editable by providing a <map-title> for the extent.

The user could click-to-edit the text of the layer/sub-layer in the layer control, or use the keyboard to activate the edit function somehow tbd how exactly.