Implement quality levels for actors & corresponding setting.

An actor file, as referenced by the VisualActor, can now define
different actors for different "quality level" setting.
In this initial version, the quality is handled directly by the object
manager.

Actor format impact:
- '<qualitylevels>' may be used as the root node, containing actor nodes
as children.
  - such actor nodes can refer to a file, or to an inline actor, or
simply be inlined.
  - such actor nodes may have a 'quality' attribute, specifying the
maximum quality level of this actor. By default, 255 (the maximum) is
implied.
- The actor format remains valid, but 'groups', 'variants', 'material',
'castshadow' and 'float' can be given a [minquality, maxquality[ range
via XML attributes. Outside of this range, the XML node is ignored
(making it possible to define, in a single actor file, several quality
levels).

Quality is a 0-255 value, with:
- Range 0-99 intended for lower level-of-detail actors (billboards,
etc.)
- Range 100-200 the 'normal' range for models. 100 is "low", 150
"medium", and 200 "high".
- Range 201-255 used for higher quality actors that might be used for
e.g. cinematics.
The range is wide to make it easier to add intermediate levels in the
future and it seemed easier given that an integer value of some kind was
required anyways.

Engine impacts:
- A new CActorDef class is introduced, wrapping an art/actors XML file
and its different quality levels. ObjectBase remains the definition of a
given 'actor', now at a given quality level.
- CActorDef imposes a maximal # of quality level for a particular actor
definition (5 currently).
- CUnit is made to refer to an Actor Definition explicitly, not a
particular ObjectBase.
- As a minor optimisation, variation keys are calculated on
pointer-to-sets-of-selections, instead of raw sets-of-selections, as
this reduces copying.
- some refactoring, including better const-correctness and hotloading
support via std::shared_ptr.

Differential Revision: https://code.wildfiregames.com/D3787
This was SVN commit r25210.

source/tools/atlas/AtlasUI/CustomControls/EditableListCtrl/FieldEditCtrl.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2009 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -81,6 +81,21 @@ void FieldEditCtrl_Color::StartEdit(wxWindow* parent, wxRect WXUNUSED(rect), lon
 
 //////////////////////////////////////////////////////////////////////////
 
+void FieldEditCtrl_Boolean::StartEdit(wxWindow* parent, wxRect rect, long row, int col)
+{
+	wxArrayString choices;
+
+	// The famous three-valued boolean.
+	choices.Add("true");
+	choices.Add("false");
+	choices.Add("");
+
+	ListCtrlValidator validator((EditableListCtrl*)parent, row, col);
+	new QuickComboBox(parent, rect, choices, validator);
+}
+
+//////////////////////////////////////////////////////////////////////////
+
 FieldEditCtrl_List::FieldEditCtrl_List(const char* listType)
 	: m_ListType(listType)
 {