Serialization: If field doesn't exist when deserialiazing, don't set it

The field could be removed as part of the class definition change.
Terrain: Don't use mapping in terrain partition

Don't need to use mapping in terrain partition because the relation
is linear so no need to save it to dictionary.
script: Once more rewrite Editor/Runtime saving/loading

We have only one class hierarchy and no editor specific classes. This
is handled by editor only methods which uses interfaces for editor
only classed which are implemented only as forwarders to
UnityEditor classes.
editor: PrefabManagerEditor add solo method for creating from resources

We need also methods to create prefabs from any asset folder, not
only from Resources one.
script: Save scene also to xml as backup

When custom serialization fail for some incompatible class definition
changes, it is nice to have scene in xml from which it could be
recreated.
terrain: Refactor old terrain clearing to solo method
script: Rename IAssetDatabase to IEditor

Better reflects purpose of interface as it is editor-wide interface.
terrain: Flush stream when saving to xml
script: Add NonMoveableObject constructor for string

Needed for serialization.
terrain: Refactor current scene path finding to solo function in editor
script: Implement level loading from XML
terrain: Fix GetDirection() for vertical partition crossing
cursor: Update cursor position only if mouse was moved
Terrain: Add variant as parameter to TerrainTile constructor

This also fixes the variant not loading from xml bug .
Terrain: Don't mark m_NonMoveableMap as [NonSerialized]

We want it to be serialized. Otherwise builtin save/load would lead
to data loss.
Level: When writing level xml, make the backup
#region renames/addition
script: Update the cursor info from LevelManager

Better to use one main Update() function when all dependent stuff is
done and use solo Update() functions for other independent game
objects.
script: Add documentation to Cursor
material: Move Border material to global assets