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.
* Add always zoom invariant and always on top rendering to
ViewProviderDatumCS
* Add a new utility class AxisOrigin for more efficient axis rendering.
Also exposed to python.
* Change ViewProviderPlacement to use AxisOrigin for rendering.
A single instance of AxisOrigin is shared by all
ViewProviderPlacement. Selection context is used to distinguish among
different instances.
