During the LinkMerge, a new icon
`Std_CoordinateSystem.svg` was used
for the `App::Origin` object that appears in
`PartDesign::Body`. However, this icon
doesn't look as good as the original `CoordinateSystem.svg`.
This commit renames the old icon to the new name. The newer icon
is kept as an alternate version `Std_CoordinateSystem_alt.svg`.
In addition, a new icon `Std_Placement.svg`
is provided for the `App::Placement` object, which previously
was also using `CoordinateSystem.svg`.
In this way, `App::Placement` and `App::Origin`
use different icons and there is no confusion
in the tree view.
According to the standard, entities of type IfcSpatialStructureElement
should not be related to other spatial structure elements through
IfcRelContainedInSpatialStructure but through IfcRelAggregates.