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.

src/scenedit/SConscript

@@ -10,6 +10,7 @@ scened_sources = Split("""
 	scen.keydlgs.cpp
 	scen.locpicker.cpp
 	scen.main.cpp
+	scen.sdfpicker.cpp
 	scen.townout.cpp
 	../view_dialogs.cpp
 	../fileio/fileio_party.cpp