Add an SDF picker for selecting a stuff done flag and optionally giving it a name.

In effect, this is a combination of two of the previous pickers:
the location picker, and the editable string picker.

This required quite a significant rework of how the tilemap places its children.

Currently it's only used in special node editing.
I plan to add its use in many other places too though.

rsrc/schemas/dialog.xsd

@@ -307,6 +307,7 @@
 			<xs:element name="button" type="button"/>
 			<xs:element name="led" type="led"/>
 			<xs:element name="group" type="ledGroup"/>
+			<xs:element name="mapgroup" type="ledGroup"/>
 			<xs:element name="slider" type="slider"/>
 		</xs:choice>
 		<xs:attribute name="name" type="xs:token"/>