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GeometryExtension was originally designed to be serializable (save/restore). However,
GeometryMigrationExtension and ViewProviderSketchGeometryExtension have demonstrated the
usefulness of runtime-only GeometryExtensions.
The problem with runtime-only extensions being of a serializable type (GeometryExtension before
this commit) arises when Part::Geometry is to serialize the extensions and serialise the number
of extensions the geometry has. If runtime-only GeometryExtensions do not save a tag, then the
restore mechanism will expect more extensions that are actually in the XML file.
This commit makes GeometryExtension by default not serializable and deriving directly from
Base::BaseClass. A new class GeometryPersistentExtension is created, deriving from GeometryExtension
to provide serialisation. All Extensions requiring serialization derive from GeometryPersistentExtension
and those not requiring it from GeometryExtension directly.
This commit adapts all extensions to the new situation:
Part: GeometryDefaultExtension
Part: GeometryMigrationExtension
Sketcher: ViewProvierSketchGeometryExtension
Sketcher: SketchGeometryExtension and ExternalGeometryExtension
