The previous icon did not follow the general style and colors
of other icons in the workbench.
If there is no `Stencil` in the `CompoundFilter` object, raise
a `ValueError` exception when the `FilterType` is `'collision-pass'`
or `'window-distance'`.
Raise an exception when `items` is empty, or has a malformed string,
when `Filtertype` is `'specific items'`.
Fix the `getNullShapeShape` function to return a simple
`Part.Shape`, so that there is no error raised if `_nullShapeShape`
doesn't exist. This function doesn't work at all. It was probably
a prototype which was never fully developed; it may be removed
completely in the future.
The docstrings for the commands `CompoundFilter` and `ExplodeCompound`
were revised.
Clean up the spacing of the code so that lines aren't very long.
The path to the icons is added to `AppPartGui.cpp`
using `Gui::BitmapFactory().addPath()`, so the icons are found
automatically in their new directories.
